diff --git a/docs/conformance/cql.md b/docs/conformance/cql.md
index 6d1b24564..2d8e89517 100644
--- a/docs/conformance/cql.md
+++ b/docs/conformance/cql.md
@@ -85,7 +85,7 @@ The section numbers refer to the documentation of the [ELM Specification](https:
 |------|---------------|------------|---------------------------------------------|-------|
 | 9.1. | ExpressionDef | ✓          |                                             |       |
 | 9.2. | ExpressionRef | !          | only inside same library                    |       |
-| 9.3. | FunctionDef   | ✗          |                                             |       |
+| 9.3. | FunctionDef   | ✓          |                                             |       |
 | 9.4. | FunctionRef   | !          | hard coded implementation of some functions |       |
 
 ### 10. Queries
@@ -322,38 +322,38 @@ The section numbers refer to the documentation of the [ELM Specification](https:
 ### 22. Type Operators
 
 | Num    | Group              | Expression | State         | Notes |
-|--------|--------------------|-----------|---------------|-------|
-| 22.1.  | As                 | !         | no strictness |       |
-| 22.2.  | CanConvert         | ✗         |               |       |
-| 22.3.  | CanConvertQuantity | ✓         |               |       |
-| 22.4.  | Children           | ✓         |               |       |
-| 22.5.  | Convert            | ✗         |               |       |
-| 22.6.  | ConvertQuantity    | ✓         |               |       |
-| 22.7.  | ConvertsToBoolean  | ✓         |               |       |
-| 22.8.  | ConvertsToDate     | ✓         |               |       |
-| 22.9.  | ConvertsToDateTime | ✓         |               |       |
-| 22.10. | ConvertsToDecimal  | ✓         |               |       |
-| 22.11. | ConvertsToLong     | ✓         |               |       |
-| 22.12. | ConvertsToInteger  | ✓         |               |       |
-| 22.13. | ConvertsToQuantity | ✓         |               |       |
-| 22.14. | ConvertsToRatio    | ✗         |               |       |
-| 22.15. | ConvertsToString   | ✓         |               |       |
-| 22.16. | ConvertsToTime     | ✓         |               |       |
-| 22.17. | Descendents        | ✓         |               |       |
-| 22.18. | Is                 | ✗         |               |       |
-| 22.19. | ToBoolean          | ✓         |               |       |
-| 22.20. | ToChars            | ✓         |               |       |
-| 22.21. | ToConcept          | ✗         |               |       |
-| 22.22. | ToDate             | ✓         |               |       |
-| 22.23. | ToDateTime         | ✓         |               |       |
-| 22.24. | ToDecimal          | ✓         |               |       |
-| 22.25. | ToInteger          | ✓         |               |       |
-| 22.26. | ToList             | ✓         |               |       |
-| 22.27. | ToLong             | ✓         |               |       |
-| 22.28. | ToQuantity         | ✓         |               |       |
-| 22.29. | ToRatio            | ✗         |               |       |
-| 22.30. | ToString           | ✓         |               |       |
-| 22.31. | ToTime             | ✓         |               |       |
+|--------|--------------------|------------|---------------|-------|
+| 22.1.  | As                 | !          | no strictness |       |
+| 22.2.  | CanConvert         | ✗          |               |       |
+| 22.3.  | CanConvertQuantity | ✓          |               |       |
+| 22.4.  | Children           | ✓          |               |       |
+| 22.5.  | Convert            | ✗          |               |       |
+| 22.6.  | ConvertQuantity    | ✓          |               |       |
+| 22.7.  | ConvertsToBoolean  | ✓          |               |       |
+| 22.8.  | ConvertsToDate     | ✓          |               |       |
+| 22.9.  | ConvertsToDateTime | ✓          |               |       |
+| 22.10. | ConvertsToDecimal  | ✓          |               |       |
+| 22.11. | ConvertsToLong     | ✓          |               |       |
+| 22.12. | ConvertsToInteger  | ✓          |               |       |
+| 22.13. | ConvertsToQuantity | ✓          |               |       |
+| 22.14. | ConvertsToRatio    | ✗          |               |       |
+| 22.15. | ConvertsToString   | ✓          |               |       |
+| 22.16. | ConvertsToTime     | ✓          |               |       |
+| 22.17. | Descendents        | ✓          |               |       |
+| 22.18. | Is                 | ✗          |               |       |
+| 22.19. | ToBoolean          | ✓          |               |       |
+| 22.20. | ToChars            | ✓          |               |       |
+| 22.21. | ToConcept          | ✗          |               |       |
+| 22.22. | ToDate             | ✓          |               |       |
+| 22.23. | ToDateTime         | ✓          |               |       |
+| 22.24. | ToDecimal          | ✓          |               |       |
+| 22.25. | ToInteger          | ✓          |               |       |
+| 22.26. | ToList             | ✓          |               |       |
+| 22.27. | ToLong             | ✓          |               |       |
+| 22.28. | ToQuantity         | ✓          |               |       |
+| 22.29. | ToRatio            | ✗          |               |       |
+| 22.30. | ToString           | ✓          |               |       |
+| 22.31. | ToTime             | ✓          |               |       |
 
 ### 23. Clinical Operators
 
diff --git a/modules/cql/src/blaze/elm/compiler/core.clj b/modules/cql/src/blaze/elm/compiler/core.clj
index 0bdc4e32c..6d5521da1 100644
--- a/modules/cql/src/blaze/elm/compiler/core.clj
+++ b/modules/cql/src/blaze/elm/compiler/core.clj
@@ -41,7 +41,11 @@
 
 
 (defmulti compile*
-  "Compiles `expression` in `context`."
+  "Compiles `expression` in `context`.
+
+  Context consists of:
+  * :library - the library in it's ELM form
+  * :node - the database node"
   {:arglists '([context expression])}
   (fn [_ {:keys [type] :as expr}]
     (assert (string? type) (format "Missing :type in expression `%s`." (pr-str expr)))
diff --git a/modules/cql/src/blaze/elm/compiler/external_data.clj b/modules/cql/src/blaze/elm/compiler/external_data.clj
index e659c46aa..0693b3c5a 100644
--- a/modules/cql/src/blaze/elm/compiler/external_data.clj
+++ b/modules/cql/src/blaze/elm/compiler/external_data.clj
@@ -18,7 +18,9 @@
 (defrecord CompartmentListRetrieveExpression [context data-type]
   core/Expression
   (-eval [_ {:keys [db]} {:keys [id]} _]
-    (d/list-compartment-resource-handles db context id data-type)))
+    (d/list-compartment-resource-handles db context id data-type))
+  (-form [_]
+    `(~'compartment-list-retrieve ~data-type)))
 
 
 (defrecord CompartmentQueryRetrieveExpression [query data-type clauses]
@@ -93,7 +95,9 @@
 (defrecord ResourceRetrieveExpression []
   core/Expression
   (-eval [_ _ resource _]
-    [resource]))
+    [resource])
+  (-form [_]
+    (list 'retrieve-resource)))
 
 
 (def ^:private resource-expr
diff --git a/modules/cql/src/blaze/elm/compiler/function.clj b/modules/cql/src/blaze/elm/compiler/function.clj
new file mode 100644
index 000000000..ece770ad5
--- /dev/null
+++ b/modules/cql/src/blaze/elm/compiler/function.clj
@@ -0,0 +1,12 @@
+(ns blaze.elm.compiler.function
+  (:require
+    [blaze.elm.compiler.core :as core]))
+
+
+(defn arity-n [name fn-expr operand-names operands]
+  (reify core/Expression
+    (-eval [_ context resource scope]
+      (let [values (map #(core/-eval % context resource scope) operands)]
+        (core/-eval fn-expr context resource (merge scope (zipmap operand-names values)))))
+    (-form [_]
+      `(~'call ~name ~@(map core/-form operands)))))
diff --git a/modules/cql/src/blaze/elm/compiler/library.clj b/modules/cql/src/blaze/elm/compiler/library.clj
index f78367791..6b5d5bbc6 100644
--- a/modules/cql/src/blaze/elm/compiler/library.clj
+++ b/modules/cql/src/blaze/elm/compiler/library.clj
@@ -1,39 +1,57 @@
 (ns blaze.elm.compiler.library
   (:require
-    [blaze.anomaly :as ba :refer [when-ok]]
+    [blaze.anomaly :as ba :refer [if-ok when-ok]]
     [blaze.elm.compiler :as compiler]
+    [blaze.elm.compiler.function :as function]
     [blaze.elm.deps-infer :as deps-infer]
     [blaze.elm.equiv-relationships :as equiv-relationships]
     [blaze.elm.normalizer :as normalizer]))
 
 
 (defn- compile-expression-def
-  "Compiles the expression of `expression-def` in `context` and associates the
-  resulting compiled expression under ::compiler/expression to the
-  `expression-def` which itself is returned.
-
-  Returns an anomaly on errors."
-  {:arglists '([context expression-def])}
-  [context {:keys [expression] :as expression-def}]
-  (let [context (assoc context :eval-context (:context expression-def))]
-    (-> (ba/try-anomaly
-          (assoc expression-def
-            ::compiler/expression (compiler/compile context expression)))
+  "Compiles the expression of `def` in `context` and returns a tuple of
+  `[name compiled-expression]` or an anomaly on errors."
+  [context {:keys [name expression] :as def}]
+  (let [context (assoc context :eval-context (:context def))]
+    (-> (ba/try-anomaly [name (compiler/compile context expression)])
         (ba/exceptionally
           #(assoc % :context context :elm/expression expression)))))
 
 
-(defn- expr-defs [context library]
+(defn- compile-function-def
+  "Compiles the function of `def` in `context`.
+
+  Returns the compiled function or an anomaly on errors."
+  [context {:keys [name operand] :as def}]
+  (when-ok [[_ expression] (compile-expression-def context def)]
+    (partial function/arity-n name expression (mapv :name operand))))
+
+
+(defn- compile-function-defs [context library]
   (transduce
-    (comp (map (partial compile-expression-def context))
-          (halt-when ba/anomaly?))
+    (filter (comp #{"FunctionDef"} :type))
     (completing
-      (fn [r {:keys [name] ::compiler/keys [expression]}]
-        (assoc r name expression)))
-    {}
+      (fn [context {:keys [name] :as def}]
+        (if-ok [function (compile-function-def context def)]
+          (assoc-in context [:functions name] function)
+          reduced)))
+    context
     (-> library :statements :def)))
 
 
+(defn- expression-defs [context library]
+  (when-ok [context (compile-function-defs context library)]
+    (transduce
+      (comp (filter (comp nil? :type))
+            (map (partial compile-expression-def context))
+            (halt-when ba/anomaly?))
+      (completing
+        (fn [r [name expression]]
+          (assoc r name expression)))
+      {}
+      (-> library :statements :def))))
+
+
 (defn- compile-parameter-def
   "Compiles the default value of `parameter-def` in `context` and associates the
   resulting compiled default value under :default to the `parameter-def` which
@@ -71,7 +89,7 @@
                     equiv-relationships/find-equiv-rels-library
                     deps-infer/infer-library-deps)
         context (assoc opts :node node :library library)]
-    (when-ok [expr-defs (expr-defs context library)
+    (when-ok [expression-defs (expression-defs context library)
               parameter-default-values (parameter-default-values context library)]
-      {:compiled-expression-defs expr-defs
+      {:compiled-expression-defs expression-defs
        :parameter-default-values parameter-default-values})))
diff --git a/modules/cql/src/blaze/elm/compiler/queries.clj b/modules/cql/src/blaze/elm/compiler/queries.clj
index 1abcfc388..9aa10f81b 100644
--- a/modules/cql/src/blaze/elm/compiler/queries.clj
+++ b/modules/cql/src/blaze/elm/compiler/queries.clj
@@ -298,18 +298,6 @@
     (throw (Exception. (str "Unsupported number of " (count sources) " sources in query.")))))
 
 
-;; ?.? IdentifierRef
-;;
-;; The IdentifierRef type defines an expression that references an identifier
-;; that is either unresolved, or has been resolved to an attribute in an
-;; unambiguous iteration scope such as a sort. Implementations should attempt to
-;; resolve the identifier, only throwing an error at compile-time (or run-time
-;; for an interpretive system) if the identifier reference cannot be resolved.
-(defmethod core/compile* :elm.compiler.type/identifier-ref
-  [_ {:keys [name]}]
-  (structured-values/->SingleScopePropertyExpression (keyword name)))
-
-
 ;; 10.3. AliasRef
 ;;
 ;; The AliasRef expression allows for the reference of a specific source within
@@ -336,7 +324,13 @@
     (->AliasRefExpression name)))
 
 
-;; 10.12. With
+;; 10.7 IdentifierRef
+(defmethod core/compile* :elm.compiler.type/identifier-ref
+  [_ {:keys [name]}]
+  (structured-values/->SingleScopePropertyExpression (keyword name)))
+
+
+;; 10.14. With
 ;;
 ;; The With clause restricts the elements of a given source to only those
 ;; elements that have elements in the related source that satisfy the suchThat
@@ -386,4 +380,4 @@
         (format "Unsupported call without single query scope.")))))
 
 
-;; TODO 10.13. Without
+;; TODO 10.15. Without
diff --git a/modules/cql/src/blaze/elm/compiler/reusing_logic.clj b/modules/cql/src/blaze/elm/compiler/reusing_logic.clj
index 8100b5678..e3a074302 100644
--- a/modules/cql/src/blaze/elm/compiler/reusing_logic.clj
+++ b/modules/cql/src/blaze/elm/compiler/reusing_logic.clj
@@ -40,11 +40,17 @@
     `(~'expr-ref ~name)))
 
 
-(defn- find-expression-def
-  "Returns the expression-def with `name` from `library` or nil if not found."
+(defn- find-def
+  "Returns the def with `name` from `library` or nil if not found."
   {:arglists '([library name])}
-  [{{expr-defs :def} :statements} name]
-  (some #(when (= name (:name %)) %) expr-defs))
+  [{{defs :def} :statements} name]
+  (some #(when (= name (:name %)) %) defs))
+
+
+(defn- find-expression-def [library name]
+  (when-let [def (find-def library name)]
+    (when (nil? (:type def))
+      def)))
 
 
 (defn- expression-def-not-found-anom [context name]
@@ -157,6 +163,18 @@
     `(~'call "ToInterval" ~(core/-form operand))))
 
 
+(defn- function-def-not-found-anom [context name]
+  (ba/incorrect
+    (format "Function definition `%s` not found." name)
+    :context context))
+
+
+(defn compile-function [{:keys [functions] :as context} name operands]
+  (if-let [function (get functions name)]
+    (function operands)
+    (throw-anom (function-def-not-found-anom context name))))
+
+
 ;; 9.4. FunctionRef
 (defmethod core/compile* :elm.compiler.type/function-ref
   [context {:keys [name] operands :operand}]
@@ -184,4 +202,14 @@
       "ToInterval"
       (->ToIntervalFunctionExpression (first operands))
 
-      (throw (Exception. (str "Unsupported function `" name "` in `FunctionRef` expression."))))))
+      (compile-function context name operands))))
+
+
+;; 9.5 OperandRef
+(defmethod core/compile* :elm.compiler.type/operand-ref
+  [_ {:keys [name]}]
+  (reify core/Expression
+    (-eval [_ _ _ scope]
+      (scope name))
+    (-form [_]
+      `(~'operand-ref ~name))))
diff --git a/modules/cql/src/blaze/elm/compiler/spec.clj b/modules/cql/src/blaze/elm/compiler/spec.clj
index 9fab2fc57..896730d14 100644
--- a/modules/cql/src/blaze/elm/compiler/spec.clj
+++ b/modules/cql/src/blaze/elm/compiler/spec.clj
@@ -10,5 +10,9 @@
   core/expr?)
 
 
+(s/def :blaze.elm.compiler/function
+  fn?)
+
+
 (s/def :elm/compile-context
   (s/keys :req-un [:elm/library :blaze.db/node]))
diff --git a/modules/cql/src/blaze/elm/spec.clj b/modules/cql/src/blaze/elm/spec.clj
index 87fa7ffef..61d35e8cf 100644
--- a/modules/cql/src/blaze/elm/spec.clj
+++ b/modules/cql/src/blaze/elm/spec.clj
@@ -622,8 +622,8 @@
   (s/keys :opt-un [:elm/name :elm/libraryName :elm.nary-expression/operand]))
 
 
-;; ?.? IdentifierRef
-(defmethod expression :elm.spec.type/identifier-ref [_]
+;; 9.5 OperandRef
+(defmethod expression :elm.spec.type/operand-ref [_]
   (s/keys :opt-un [:elm/name]))
 
 
@@ -669,14 +669,23 @@
      :elm.sort-by-item.by-expression/expression]))
 
 
-;; 10.9. RelationshipClause
+;; 10.7 IdentifierRef
+(defmethod expression :elm.spec.type/identifier-ref [_]
+  (s/keys :req-un [:elm/name] :opt-un [:elm/libraryName]))
+
+
+;; TODO: 10.8. LetClause
+
+;; TODO 10.9. QueryLetRef
+
+;; 10.10. RelationshipClause
 (defmulti relationship-clause :type)
 
 (s/def :elm/relationship-clause
   (s/multi-spec relationship-clause :type))
 
 
-;; 10.10. ReturnClause
+;; 10.11. ReturnClause
 (s/def :elm.return-clause/expression
   :elm/expression)
 
@@ -690,9 +699,9 @@
           :opt-un [:elm.return-clause/distinct]))
 
 
-;; TODO: 10.11. AggregateClause
+;; TODO: 10.12. AggregateClause
 
-;; 10.12. SortClause
+;; 10.13. SortClause
 (s/def :elm.sort-clause/by
   (s/coll-of :elm/sort-by-item :min-count 1))
 
@@ -701,7 +710,7 @@
   (s/keys :req-un [:elm.sort-clause/by]))
 
 
-;; 10.13. With
+;; 10.14. With
 (defmethod relationship-clause "With" [_]
   (s/keys :req-un [:elm/expression :elm/alias :elm.query/suchThat]))
 
@@ -711,7 +720,7 @@
           :opt-un [:elm.query/suchThat]))
 
 
-;; 10.14. Without
+;; 10.15. Without
 (defmethod relationship-clause "Without" [_]
   (s/keys :req-un [:elm/expression :elm/alias :elm.query/suchThat]))
 
diff --git a/modules/cql/test/blaze/elm/compiler/external_data_test.clj b/modules/cql/test/blaze/elm/compiler/external_data_test.clj
index d6d38bed8..129077f8f 100644
--- a/modules/cql/test/blaze/elm/compiler/external_data_test.clj
+++ b/modules/cql/test/blaze/elm/compiler/external_data_test.clj
@@ -75,9 +75,13 @@
               db (d/db node)
               patient (d/resource-handle db "Patient" "0")]
 
-          (given (core/-eval expr {:db db} patient nil)
-            [0 fhir-spec/fhir-type] := :fhir/Patient
-            [0 :id] := "0"))))
+          (testing "eval"
+            (given (core/-eval expr {:db db} patient nil)
+              [0 fhir-spec/fhir-type] := :fhir/Patient
+              [0 :id] := "0"))
+
+          (testing "form"
+            (is (= '(retrieve-resource) (core/-form expr)))))))
 
     (testing "Observation"
       (with-system-data [{:blaze.db/keys [node]} mem-node-system]
@@ -94,9 +98,13 @@
               db (d/db node)
               patient (d/resource-handle db "Patient" "0")]
 
-          (given (core/-eval expr {:db db} patient nil)
-            [0 fhir-spec/fhir-type] := :fhir/Observation
-            [0 :id] := "1")))
+          (testing "eval"
+            (given (core/-eval expr {:db db} patient nil)
+              [0 fhir-spec/fhir-type] := :fhir/Observation
+              [0 :id] := "1"))
+
+          (testing "form"
+            (is (= '(compartment-list-retrieve "Observation") (core/-form expr))))))
 
       (testing "with one code"
         (with-system-data [{:blaze.db/keys [node]} mem-node-system]
@@ -107,10 +115,10 @@
             [:put {:fhir/type :fhir/Observation :id "1"
                    :code
                    #fhir/CodeableConcept
-                       {:coding
-                        [#fhir/Coding
-                            {:system #fhir/uri"system-192253"
-                             :code #fhir/code"code-192300"}]}
+                           {:coding
+                            [#fhir/Coding
+                                    {:system #fhir/uri"system-192253"
+                                     :code #fhir/code"code-192300"}]}
                    :subject
                    #fhir/Reference{:reference "Patient/0"}}]]]
 
@@ -123,9 +131,9 @@
                    [{:name "sys-def-131750"
                      :id "system-192253"}]}}}
                 elm #elm/retrieve
-                        {:type "Observation"
-                         :codes #elm/list [#elm/code ["sys-def-131750"
-                                                    "code-192300"]]}
+                            {:type "Observation"
+                             :codes #elm/list [#elm/code ["sys-def-131750"
+                                                          "code-192300"]]}
                 expr (c/compile context elm)
                 db (d/db node)
                 patient (d/resource-handle db "Patient" "0")]
@@ -149,19 +157,19 @@
             [:put {:fhir/type :fhir/Observation :id "1"
                    :code
                    #fhir/CodeableConcept
-                       {:coding
-                        [#fhir/Coding
-                            {:system #fhir/uri"system-192253"
-                             :code #fhir/code"code-192300"}]}
+                           {:coding
+                            [#fhir/Coding
+                                    {:system #fhir/uri"system-192253"
+                                     :code #fhir/code"code-192300"}]}
                    :subject
                    #fhir/Reference{:reference "Patient/0"}}]
             [:put {:fhir/type :fhir/Observation :id "2"
                    :code
                    #fhir/CodeableConcept
-                       {:coding
-                        [#fhir/Coding
-                            {:system #fhir/uri"system-192253"
-                             :code #fhir/code"code-140541"}]}
+                           {:coding
+                            [#fhir/Coding
+                                    {:system #fhir/uri"system-192253"
+                                     :code #fhir/code"code-140541"}]}
                    :subject
                    #fhir/Reference{:reference "Patient/0"}}]]]
 
@@ -174,10 +182,10 @@
                    [{:name "sys-def-131750"
                      :id "system-192253"}]}}}
                 elm #elm/retrieve
-                        {:type "Observation"
-                         :codes
-                         #elm/list [#elm/code ["sys-def-131750" "code-192300"]
-                                   #elm/code ["sys-def-131750" "code-140541"]]}
+                            {:type "Observation"
+                             :codes
+                             #elm/list [#elm/code ["sys-def-131750" "code-192300"]
+                                        #elm/code ["sys-def-131750" "code-140541"]]}
                 expr (c/compile context elm)
                 db (d/db node)
                 patient (d/resource-handle db "Patient" "0")]
@@ -215,10 +223,10 @@
         [[[:put {:fhir/type :fhir/Medication :id "0"
                  :code
                  #fhir/CodeableConcept
-                     {:coding
-                      [#fhir/Coding
-                          {:system #fhir/uri"system-225806"
-                           :code #fhir/code"code-225809"}]}}]]]
+                         {:coding
+                          [#fhir/Coding
+                                  {:system #fhir/uri"system-225806"
+                                   :code #fhir/code"code-225809"}]}}]]]
 
         (let [context
               {:node node
@@ -229,9 +237,9 @@
                  [{:name "sys-def-225944"
                    :id "system-225806"}]}}}
               elm #elm/retrieve
-                      {:type "Medication"
-                       :codes #elm/list [#elm/code ["sys-def-225944"
-                                                  "code-225809"]]}
+                          {:type "Medication"
+                           :codes #elm/list [#elm/code ["sys-def-225944"
+                                                        "code-225809"]]}
               expr (c/compile context elm)
               db (d/db node)]
 
@@ -251,10 +259,10 @@
                  [{:name "sys-def-225944"
                    :id "system-225806"}]}}}
               elm #elm/retrieve
-                      {:type "Medication"
-                       :codes #elm/list [#elm/code ["sys-def-225944"
-                                                  "code-225809"]]
-                       :code-property "foo"}]
+                          {:type "Medication"
+                           :codes #elm/list [#elm/code ["sys-def-225944"
+                                                        "code-225809"]]
+                           :code-property "foo"}]
 
           (given (ba/try-anomaly (c/compile context elm))
             ::anom/category := ::anom/not-found
@@ -270,10 +278,10 @@
                         [{:name "name-174207"
                           :resultTypeName "{http://hl7.org/fhir}Patient"}]}}
               elm #elm/retrieve
-                      {:type "Observation"
-                       :context #elm/expression-ref "name-174207"
-                       :codes #elm/list [#elm/code ["sys-def-174848"
-                                                  "code-174911"]]}
+                          {:type "Observation"
+                           :context #elm/expression-ref "name-174207"
+                           :codes #elm/list [#elm/code ["sys-def-174848"
+                                                        "code-174911"]]}
               expr (c/compile {:node node :library library} elm)]
           (given expr
             type := WithRelatedContextQueryRetrieveExpression))))
@@ -287,11 +295,11 @@
                         [{:name "name-174207"
                           :resultTypeName "{http://hl7.org/fhir}Patient"}]}}
               elm #elm/retrieve
-                      {:type "Observation"
-                       :context #elm/expression-ref "name-174207"
-                       :codes #elm/list [#elm/code ["sys-def-174848"
-                                                  "code-174911"]]
-                       :code-property "foo"}]
+                          {:type "Observation"
+                           :context #elm/expression-ref "name-174207"
+                           :codes #elm/list [#elm/code ["sys-def-174848"
+                                                        "code-174911"]]
+                           :code-property "foo"}]
           (given (ba/try-anomaly (c/compile {:node node :library library} elm))
             ::anom/category := ::anom/not-found
             ::anom/message := "The search-param with code `foo` and type `Observation` was not found.")))))
diff --git a/modules/cql/test/blaze/elm/compiler/library_test.clj b/modules/cql/test/blaze/elm/compiler/library_test.clj
index fe5924a4e..8928d6eb2 100644
--- a/modules/cql/test/blaze/elm/compiler/library_test.clj
+++ b/modules/cql/test/blaze/elm/compiler/library_test.clj
@@ -2,6 +2,7 @@
   (:require
     [blaze.cql-translator :as t]
     [blaze.db.api-stub :refer [mem-node-system]]
+    [blaze.elm.compiler :as compiler]
     [blaze.elm.compiler.library :as library]
     [blaze.elm.compiler.library-spec]
     [blaze.fhir.spec.type.system :as system]
@@ -37,23 +38,52 @@
         (given (library/compile-library node library {})
           :compiled-expression-defs := {"Foo" true}))))
 
-  (testing "one static expression"
+  (testing "one dynamic expression"
     (let [library (t/translate "library Test
-    using FHIR version '4.0.0'
-    context Patient
-    define Gender: Patient.gender")]
+      using FHIR version '4.0.0'
+      context Patient
+      define Gender: Patient.gender")]
       (with-system [{:blaze.db/keys [node]} mem-node-system]
         (given (library/compile-library node library {})
-          [:compiled-expression-defs "Gender" :key] := :gender
-          [:compiled-expression-defs "Gender" :source :name] := "Patient"))))
+          [:compiled-expression-defs "Gender" compiler/form] := '(:gender (expr-ref "Patient"))))))
 
-  (testing "with compile-time error"
+  (testing "one function"
     (let [library (t/translate "library Test
-    define Error: singleton from {1, 2}")]
+      using FHIR version '4.0.0'
+      context Patient
+      define function Gender(P Patient): P.gender
+      define InInitialPopulation: Gender(Patient)")]
       (with-system [{:blaze.db/keys [node]} mem-node-system]
         (given (library/compile-library node library {})
-          ::anom/category := ::anom/conflict
-          ::anom/message := "More than one element in `SingletonFrom` expression."))))
+          [:compiled-expression-defs "InInitialPopulation" compiler/form] := '(call "Gender" (expr-ref "Patient"))))))
+
+  (testing "two functions, one calling the other"
+    (let [library (t/translate "library Test
+      using FHIR version '4.0.0'
+      context Patient
+      define function Inc(i System.Integer): i + 1
+      define function Inc2(i System.Integer): Inc(i) + 1
+      define InInitialPopulation: Inc2(1)")]
+      (with-system [{:blaze.db/keys [node]} mem-node-system]
+        (given (library/compile-library node library {})
+          [:compiled-expression-defs "InInitialPopulation" compiler/form] := '(call "Inc2" 1)))))
+
+  (testing "with compile-time error"
+    (testing "function"
+      (let [library (t/translate "library Test
+          define function Error(): singleton from {1, 2}")]
+        (with-system [{:blaze.db/keys [node]} mem-node-system]
+          (given (library/compile-library node library {})
+            ::anom/category := ::anom/conflict
+            ::anom/message := "More than one element in `SingletonFrom` expression."))))
+
+    (testing "expression"
+      (let [library (t/translate "library Test
+        define Error: singleton from {1, 2}")]
+        (with-system [{:blaze.db/keys [node]} mem-node-system]
+          (given (library/compile-library node library {})
+            ::anom/category := ::anom/conflict
+            ::anom/message := "More than one element in `SingletonFrom` expression.")))))
 
   (testing "with parameter default"
     (let [library (t/translate "library Test
diff --git a/modules/cql/test/blaze/elm/compiler/queries_test.clj b/modules/cql/test/blaze/elm/compiler/queries_test.clj
index 4cbba0382..cee6b06bb 100644
--- a/modules/cql/test/blaze/elm/compiler/queries_test.clj
+++ b/modules/cql/test/blaze/elm/compiler/queries_test.clj
@@ -232,7 +232,21 @@
     ::result))
 
 
-;; 10.12. With
+;; 10.7 IdentifierRef
+;;
+;; The IdentifierRef type defines an expression that references an identifier
+;; that is either unresolved, or has been resolved to an attribute in an
+;; unambiguous iteration scope such as a sort. Implementations should attempt to
+;; resolve the identifier, only throwing an error at compile-time (or run-time
+;; for an interpretive system) if the identifier reference cannot be resolved.
+(deftest compile-identifier-ref-test
+  (let [expr (c/compile {} {:type "IdentifierRef" :name "foo"})]
+
+    (testing "form"
+      (is (= '(:foo default) (core/-form expr))))))
+
+
+;; 10.14. With
 (deftest compile-with-clause-test
   (testing "Equiv With with two Observations comparing there subjects."
     (with-system-data [{:blaze.db/keys [node]} mem-node-system]
diff --git a/modules/cql/test/blaze/elm/compiler/reusing_logic_test.clj b/modules/cql/test/blaze/elm/compiler/reusing_logic_test.clj
index e715f2551..0b19d9bbd 100644
--- a/modules/cql/test/blaze/elm/compiler/reusing_logic_test.clj
+++ b/modules/cql/test/blaze/elm/compiler/reusing_logic_test.clj
@@ -7,6 +7,7 @@
     [blaze.anomaly :as ba]
     [blaze.elm.compiler :as c]
     [blaze.elm.compiler.core :as core]
+    [blaze.elm.compiler.function :as function]
     [blaze.elm.compiler.test-util :as tu]
     [blaze.elm.interval :as interval]
     [blaze.elm.literal :as elm]
@@ -66,9 +67,59 @@
 ;; The FunctionRef type defines an expression that invokes a previously defined
 ;; function. The result of evaluating each operand is passed to the function.
 (deftest compile-function-ref-test
+  (testing "Throws error on missing function"
+    (given (ba/try-anomaly (c/compile {} #elm/function-ref ["name-175844"]))
+      ::anom/category := ::anom/incorrect
+      ::anom/message := "Function definition `name-175844` not found."
+      :context := {}))
+
+  (testing "Custom function with arity 0"
+    (let [function-name "name-210650"
+          fn-expr (c/compile {} #elm/integer "1")
+          compile-ctx {:functions {function-name (partial function/arity-n function-name fn-expr [])}}
+          elm (elm/function-ref [function-name])
+          expr (c/compile compile-ctx elm)]
+      (testing "eval"
+        (is (= 1 (core/-eval expr {} nil nil))))
+
+      (testing "form"
+        (is (= `(~'call ~function-name) (core/-form expr))))))
+
+  (testing "Custom function with arity 1"
+    (let [function-name "name-180815"
+          fn-expr (c/compile {} #elm/negate #elm/operand-ref"x")
+          compile-ctx {:library {:parameters {:def [{:name "a"}]}}
+                       :functions {function-name (partial function/arity-n function-name fn-expr ["x"])}}
+          elm (elm/function-ref [function-name #elm/parameter-ref "a"])
+          expr (c/compile compile-ctx elm)]
+      (testing "eval"
+        (are [a res] (= res (core/-eval expr {:parameters {"a" a}} nil nil))
+          1 -1
+          -1 1
+          0 0))
+
+      (testing "form"
+        (is (= `(~'call ~function-name (~'param-ref "a")) (core/-form expr))))))
+
+  (testing "Custom function with arity 2"
+    (let [function-name "name-184652"
+          fn-expr (c/compile {} #elm/add [#elm/operand-ref"x" #elm/operand-ref"y"])
+          compile-ctx {:library {:parameters {:def [{:name "a"} {:name "b"}]}}
+                       :functions {function-name (partial function/arity-n function-name fn-expr ["x" "y"])}}
+          elm (elm/function-ref [function-name #elm/parameter-ref "a" #elm/parameter-ref "b"])
+          expr (c/compile compile-ctx elm)]
+      (testing "eval"
+        (are [a b res] (= res (core/-eval expr {:parameters {"a" a "b" b}} nil nil))
+          1 1 2
+          1 0 1
+          0 1 1))
+
+      (testing "form"
+        (is (= `(~'call ~function-name (~'param-ref "a") (~'param-ref "b")) (core/-form expr))))))
+
   (testing "ToQuantity"
     (let [compile-ctx {:library {:parameters {:def [{:name "x"}]}}}
-          elm (elm/function-ref "ToQuantity" #elm/parameter-ref "x")
+          elm #elm/function-ref ["ToQuantity" #elm/parameter-ref "x"]
           expr (c/compile compile-ctx elm)]
       (testing "eval"
         (are [x res] (= res (core/-eval expr {:parameters {"x" x}} nil nil))
@@ -81,7 +132,7 @@
 
   (testing "ToDateTime"
     (let [compile-ctx {:library {:parameters {:def [{:name "x"}]}}}
-          elm (elm/function-ref "ToDateTime" #elm/parameter-ref "x")
+          elm #elm/function-ref ["ToDateTime" #elm/parameter-ref "x"]
           expr (c/compile compile-ctx elm)
           eval-ctx (fn [x] {:now tu/now :parameters {"x" x}})]
       (testing "eval"
@@ -98,7 +149,7 @@
 
   (testing "ToString"
     (let [compile-ctx {:library {:parameters {:def [{:name "x"}]}}}
-          elm (elm/function-ref "ToString" #elm/parameter-ref "x")
+          elm #elm/function-ref ["ToString" #elm/parameter-ref "x"]
           expr (c/compile compile-ctx elm)]
       (testing "eval"
         (are [x res] (= res (core/-eval expr {:parameters {"x" x}} nil nil))
@@ -110,29 +161,38 @@
 
   (testing "ToInterval"
     (let [compile-ctx {:library {:parameters {:def [{:name "x"}]}}}
-          elm (elm/function-ref "ToInterval" #elm/parameter-ref "x")
+          elm #elm/function-ref ["ToInterval" #elm/parameter-ref "x"]
           expr (c/compile compile-ctx elm)
           eval-ctx (fn [x] {:now tu/now :parameters {"x" x}})]
       (testing "eval"
         (are [x res] (= res (core/-eval expr (eval-ctx x) nil nil))
           #fhir/Period
-              {:start #fhir/dateTime"2021-02-23T15:12:45+01:00"
-               :end #fhir/dateTime"2021-02-23T16:00:00+01:00"}
+                  {:start #fhir/dateTime"2021-02-23T15:12:45+01:00"
+                   :end #fhir/dateTime"2021-02-23T16:00:00+01:00"}
           (interval/interval
             (system/date-time 2021 2 23 14 12 45)
             (system/date-time 2021 2 23 15 0 0))
           #fhir/Period
-              {:start nil
-               :end #fhir/dateTime"2021-02-23T16:00:00+01:00"}
+                  {:start nil
+                   :end #fhir/dateTime"2021-02-23T16:00:00+01:00"}
           (interval/interval
             nil
             (system/date-time 2021 2 23 15 0 0))
           #fhir/Period
-              {:start #fhir/dateTime"2021-02-23T15:12:45+01:00"
-               :end nil}
+                  {:start #fhir/dateTime"2021-02-23T15:12:45+01:00"
+                   :end nil}
           (interval/interval
             (system/date-time 2021 2 23 14 12 45)
             nil)))
 
       (testing "form"
         (is (= '(call "ToInterval" (param-ref "x")) (core/-form expr)))))))
+
+
+;; 9.5 OperandRef
+;;
+;; The OperandRef expression allows the value of an operand to be referenced as
+;; part of an expression within the body of a function definition.
+(deftest compile-operand-ref-test
+  (testing "form"
+    (is (= '(operand-ref "x") (core/-form (c/compile {} #elm/operand-ref"x"))))))
diff --git a/modules/cql/test/blaze/elm/literal.clj b/modules/cql/test/blaze/elm/literal.clj
index 04f3f769e..cc3f9635d 100644
--- a/modules/cql/test/blaze/elm/literal.clj
+++ b/modules/cql/test/blaze/elm/literal.clj
@@ -110,10 +110,16 @@
 
 
 ;; 9.4. FunctionRef
-(defn function-ref [name & ops]
+(defn function-ref [[name & ops]]
   {:type "FunctionRef"
    :name name
-   :operand ops})
+   :operand (vec ops)})
+
+
+;; 9.5. OperandRef
+(defn operand-ref [name]
+  {:type "OperandRef"
+   :name name})
 
 
 
diff --git a/modules/cql/test/data_readers.clj b/modules/cql/test/data_readers.clj
index 750db50ac..33ef2a785 100644
--- a/modules/cql/test/data_readers.clj
+++ b/modules/cql/test/data_readers.clj
@@ -10,6 +10,8 @@
  elm/quantity blaze.elm.literal/quantity
  elm/parameter-ref blaze.elm.literal/parameter-ref
  elm/expression-ref blaze.elm.literal/expression-ref
+ elm/function-ref blaze.elm.literal/function-ref
+ elm/operand-ref blaze.elm.literal/operand-ref
  elm/retrieve blaze.elm.literal/retrieve
  elm/equal blaze.elm.literal/equal
  elm/equivalent blaze.elm.literal/equivalent
diff --git a/modules/operation-measure-evaluate-measure/test/blaze/fhir/operation/evaluate_measure/q39-social-sec-num-query.cql b/modules/operation-measure-evaluate-measure/test/blaze/fhir/operation/evaluate_measure/q39-social-sec-num-query.cql
index c06674931..d4f62ff66 100644
--- a/modules/operation-measure-evaluate-measure/test/blaze/fhir/operation/evaluate_measure/q39-social-sec-num-query.cql
+++ b/modules/operation-measure-evaluate-measure/test/blaze/fhir/operation/evaluate_measure/q39-social-sec-num-query.cql
@@ -4,12 +4,11 @@ include FHIRHelpers version '4.0.0'
 
 context Patient
 
+define function IntegerParts(s String):
+    from (Split(s, '-')) S where ConvertsToInteger(S)
+
 define SocialSecurityNumber:
   First(Patient.identifier.where(system = 'http://hl7.org/fhir/sid/us-ssn').value)
 
-define IntegerParts:
-    from (Split(SocialSecurityNumber, '-')) S
-    where ConvertsToInteger(S)
-
 define InInitialPopulation:
-    Count(IntegerParts) = 3
+    Count(IntegerParts(SocialSecurityNumber)) = 3