diff --git a/pddl/parser/domain.lark b/pddl/parser/domain.lark
index 0b1f558..55e42fa 100644
--- a/pddl/parser/domain.lark
+++ b/pddl/parser/domain.lark
@@ -54,18 +54,18 @@ atomic_formula_term:   LPAR predicate term* RPAR
 ?predicate: NAME
 constant: NAME
 
-binary_comp: GREATER_OP
-           | LESSER_OP
-           | EQUAL_OP
-           | GREATER_EQUAL_OP
-           | LESSER_EQUAL_OP
+?binary_comp: GREATER_OP
+            | LESSER_OP
+            | EQUAL_OP
+            | GREATER_EQUAL_OP
+            | LESSER_EQUAL_OP
 
-assign_op: INCREASE
-         | DECREASE
+?assign_op: INCREASE
+          | DECREASE
 
-f_exp: NUMBER
+?f_exp: NUMBER
      | f_head
-?f_head: NAME
+f_head: NAME
        | LPAR NAME term* RPAR
 
 typed_list_variable:   variable*
diff --git a/pddl/parser/domain.py b/pddl/parser/domain.py
index c195c1c..c7c3867 100644
--- a/pddl/parser/domain.py
+++ b/pddl/parser/domain.py
@@ -29,6 +29,8 @@
     GreaterThan,
     LesserEqualThan,
     LesserThan,
+    Increase,
+    Decrease,
 )
 from pddl.logic.predicates import DerivedPredicate, EqualTo, Predicate
 from pddl.logic.terms import Constant, Variable
@@ -282,6 +284,16 @@ def cond_effect(self, args):
         assert_(len(args) == 1)
         return args[0]
 
+    def num_effect(self, args):
+        """Process the 'num_effect' rule."""
+        function = args[2]
+        value = args[3]
+        if args[1] == Symbols.INCREASE.value:
+            return Increase(function, value)
+        if args[1] == Symbols.DECREASE.value:
+            return Decrease(function, value)
+        raise PDDLParsingError("Assign operator not recognized")
+
     def atomic_formula_term(self, args):
         """Process the 'atomic_formula_term' rule."""
 
@@ -331,6 +343,14 @@ def atomic_function_skeleton(self, args):
         variables = self._formula_skeleton(args)
         return Function(function_name, *variables)
 
+    def f_head(self, args):
+        """Process the 'f_head' rule."""
+        if len(args) == 1:
+            return args[0]
+        function_name = args[1]
+        variables = [Variable(x, {}) for x in args[2:-1]]
+        return Function(function_name, *variables)
+
     def typed_list_name(self, args) -> Dict[name, Optional[name]]:
         """Process the 'typed_list_name' rule."""
         try: