diff --git a/catculator-core/src/main/scala/pl/writeonly/catculator/core/adt/calculus/Constants.scala b/catculator-core/src/main/scala/pl/writeonly/catculator/core/adt/calculus/Constants.scala
index 1e57f89..cab81bc 100644
--- a/catculator-core/src/main/scala/pl/writeonly/catculator/core/adt/calculus/Constants.scala
+++ b/catculator-core/src/main/scala/pl/writeonly/catculator/core/adt/calculus/Constants.scala
@@ -18,10 +18,9 @@ object Constants {
 
   def appK(a: CombinatorBT): Node[Combinator] = Node(kCom, a)
 
-  def app4(c1: CombinatorBT, c2: CombinatorBT, c3: CombinatorBT, c4: CombinatorBT): CombinatorBT = Node(c1, app3(c2, c3, c4))
+  def app4(c1: CombinatorBT, c2: CombinatorBT, c3: CombinatorBT, c4: CombinatorBT): CombinatorBT = Node(app3(c1, c2, c3), c4)
 
-  def app3(c1: CombinatorBT, c2: CombinatorBT, c3: CombinatorBT): CombinatorBT = Node(c1, Node(c2, c3))
+  def app3(c1: CombinatorBT, c2: CombinatorBT, c3: CombinatorBT): CombinatorBT = Node(Node(c1, c2), c3)
 
   def com(c: Combinator): CombinatorBT = Leaf(c)
-
 }
diff --git a/catculator-core/src/main/scala/pl/writeonly/catculator/core/adt/calculus/InputEncoder.scala b/catculator-core/src/main/scala/pl/writeonly/catculator/core/adt/calculus/InputEncoder.scala
index 8de318e..e4ee0a9 100644
--- a/catculator-core/src/main/scala/pl/writeonly/catculator/core/adt/calculus/InputEncoder.scala
+++ b/catculator-core/src/main/scala/pl/writeonly/catculator/core/adt/calculus/InputEncoder.scala
@@ -1,6 +1,5 @@
 package pl.writeonly.catculator.core.adt.calculus
 
-import cats.implicits.catsSyntaxEq
 import pl.writeonly.catculator.core.adt.calculus.Combinator.CombinatorBT
 import pl.writeonly.catculator.core.adt.calculus.Constants._
 import spire.math.Natural
@@ -21,9 +20,9 @@ object InputEncoder {
 
   def cons(a: CombinatorBT, b: CombinatorBT): CombinatorBT = app3(sCom, Constants.app3SI(appK(a)), appK(b))
 
-  def church(n: Natural): CombinatorBT = n.toBigInt match {
-    case n if n === BigInt(0) => falseCom
-    case n                    => succChurch(Natural(n - BigInt(1)))
+  def church(n: Natural): CombinatorBT = n match {
+    case n if n === Natural.zero => falseCom
+    case n                       => succChurch(n - Natural.one)
   }
 
   private def succChurch(n: Natural): CombinatorBT = successor(church(n))
diff --git a/catculator-core/src/main/scala/pl/writeonly/catculator/core/adt/tree/BinaryTree.scala b/catculator-core/src/main/scala/pl/writeonly/catculator/core/adt/tree/BinaryTree.scala
index fc527ba..9ca2f18 100644
--- a/catculator-core/src/main/scala/pl/writeonly/catculator/core/adt/tree/BinaryTree.scala
+++ b/catculator-core/src/main/scala/pl/writeonly/catculator/core/adt/tree/BinaryTree.scala
@@ -3,5 +3,3 @@ package pl.writeonly.catculator.core.adt.tree
 enum BinaryTree[+A]:
   case Leaf(leaf: A)
   case Node(first: BinaryTree[A], follow: BinaryTree[A])
-
-object BinaryTree {}
diff --git a/catculator-core/src/main/scala/pl/writeonly/catculator/core/reducers/LambdaReducer.scala b/catculator-core/src/main/scala/pl/writeonly/catculator/core/reducers/LambdaReducer.scala
index fc98729..03f73e8 100644
--- a/catculator-core/src/main/scala/pl/writeonly/catculator/core/reducers/LambdaReducer.scala
+++ b/catculator-core/src/main/scala/pl/writeonly/catculator/core/reducers/LambdaReducer.scala
@@ -1,5 +1,7 @@
 package pl.writeonly.catculator.core.reducers
 
+import cats.Applicative
+import cats.implicits._
 import pl.writeonly.catculator.core.adt.calculus.Combinator._
 import pl.writeonly.catculator.core.adt.calculus.Lambda
 import pl.writeonly.catculator.core.adt.calculus.Lambda._
@@ -7,11 +9,8 @@ import pl.writeonly.catculator.core.adt.tree.BinaryTree._
 
 object LambdaReducer {
   def toCombinatorBT(l: Lambda): Either[Lambda, CombinatorBT] = l match {
-    case Com(c) => Right(Leaf(c))
-    case App(f, x) => for {
-        fc <- toCombinatorBT(f)
-        xc <- toCombinatorBT(x)
-      } yield Node(fc, xc)
-    case _ => Left(l)
+    case Com(c)    => Right(Leaf(c))
+    case App(f, x) => Applicative[Either[Lambda, *]].map2(toCombinatorBT(f), toCombinatorBT(x))(Node.apply)
+    case _         => Left(l)
   }
 }
diff --git a/catculator-core/src/test/scala/pl/writeonly/catculator/core/adt/calculus/InputEncoderSpec.scala b/catculator-core/src/test/scala/pl/writeonly/catculator/core/adt/calculus/InputEncoderSpec.scala
index 6466b37..2bc8f72 100644
--- a/catculator-core/src/test/scala/pl/writeonly/catculator/core/adt/calculus/InputEncoderSpec.scala
+++ b/catculator-core/src/test/scala/pl/writeonly/catculator/core/adt/calculus/InputEncoderSpec.scala
@@ -8,8 +8,8 @@ import spire.math.Natural
 
 class InputEncoderSpec extends TableDrivenPropertySpec {
 
-  private val oneCom = Node(Leaf(S), Node(Node(Leaf(S), Node(Node(Leaf(K), Leaf(S)), Leaf(K))), Node(Leaf(K), Leaf(I))))
-  private val zeroListCom = Node(Leaf(S), Node(Node(Leaf(S), Node(Leaf(I), Node(Leaf(K), Node(Leaf(K), Leaf(I))))), Node(Leaf(K), Node(Leaf(K), Leaf(I)))))
+  private val oneCom = Node(Node(Leaf(S), Node(Node(Leaf(S), Node(Leaf(K), Leaf(S))), Leaf(K))), Node(Leaf(K), Leaf(I)))
+  private val zeroListCom = Node(Node(Leaf(S), Node(Node(Leaf(S), Leaf(I)), Node(Leaf(K), Node(Leaf(K), Leaf(I))))), Node(Leaf(K), Node(Leaf(K), Leaf(I))))
 
   it should "church number" in {