diff --git a/core/src/main/kotlin/org/nwapw/abacus/tree/BinaryNode.kt b/core/src/main/kotlin/org/nwapw/abacus/tree/BinaryNode.kt
index 7505f82..01efa8d 100644
--- a/core/src/main/kotlin/org/nwapw/abacus/tree/BinaryNode.kt
+++ b/core/src/main/kotlin/org/nwapw/abacus/tree/BinaryNode.kt
@@ -11,7 +11,7 @@ package org.nwapw.abacus.tree
  * @param left the left node.
  * @param right the right node.
  */
-data class BinaryNode(val operation: String, val left: TreeNode? = null, val right: TreeNode?) : TreeNode() {
+class BinaryNode(val operation: String, val left: TreeNode? = null, val right: TreeNode?) : TreeNode() {
 
     override fun <T : Any> reduce(reducer: Reducer<T>): T? {
         val leftReduce = left?.reduce(reducer) ?: return null
diff --git a/core/src/main/kotlin/org/nwapw/abacus/tree/FunctionNode.kt b/core/src/main/kotlin/org/nwapw/abacus/tree/FunctionNode.kt
index 64c1298..dcc8b09 100644
--- a/core/src/main/kotlin/org/nwapw/abacus/tree/FunctionNode.kt
+++ b/core/src/main/kotlin/org/nwapw/abacus/tree/FunctionNode.kt
@@ -8,7 +8,7 @@ package org.nwapw.abacus.tree
  *
  * @param function the function string.
  */
-data class FunctionNode(val function: String) : TreeNode() {
+class FunctionNode(val function: String) : TreeNode() {
 
     /**
      * List of function parameters added to this node.
diff --git a/core/src/main/kotlin/org/nwapw/abacus/tree/NumberNode.kt b/core/src/main/kotlin/org/nwapw/abacus/tree/NumberNode.kt
index de6597a..624a50a 100644
--- a/core/src/main/kotlin/org/nwapw/abacus/tree/NumberNode.kt
+++ b/core/src/main/kotlin/org/nwapw/abacus/tree/NumberNode.kt
@@ -10,7 +10,7 @@ import org.nwapw.abacus.number.NumberInterface
  *
  * @number the number value of this node.
  */
-data class NumberNode(val number: String) : TreeNode() {
+class NumberNode(val number: String) : TreeNode() {
 
     override fun <T : Any> reduce(reducer: Reducer<T>): T? {
         return reducer.reduceNode(this)
diff --git a/core/src/main/kotlin/org/nwapw/abacus/tree/UnaryNode.kt b/core/src/main/kotlin/org/nwapw/abacus/tree/UnaryNode.kt
index 65853ba..c2375b2 100644
--- a/core/src/main/kotlin/org/nwapw/abacus/tree/UnaryNode.kt
+++ b/core/src/main/kotlin/org/nwapw/abacus/tree/UnaryNode.kt
@@ -9,7 +9,7 @@ package org.nwapw.abacus.tree
  * @param operation the operation applied to the given node.
  * @param applyTo the node to which the operation will be applied.
  */
-data class UnaryNode(val operation: String, val applyTo: TreeNode? = null) : TreeNode() {
+class UnaryNode(val operation: String, val applyTo: TreeNode? = null) : TreeNode() {
 
     override fun <T : Any> reduce(reducer: Reducer<T>): T? {
         val reducedChild = applyTo?.reduce(reducer) ?: return null
diff --git a/core/src/main/kotlin/org/nwapw/abacus/tree/VariableNode.kt b/core/src/main/kotlin/org/nwapw/abacus/tree/VariableNode.kt
index 0bf05c6..93d91dc 100644
--- a/core/src/main/kotlin/org/nwapw/abacus/tree/VariableNode.kt
+++ b/core/src/main/kotlin/org/nwapw/abacus/tree/VariableNode.kt
@@ -8,7 +8,7 @@ package org.nwapw.abacus.tree
  *
  * @param variable the actual variable name that this node represents.
  */
-data class VariableNode(val variable: String) : TreeNode() {
+class VariableNode(val variable: String) : TreeNode() {
 
     override fun <T : Any> reduce(reducer: Reducer<T>): T? {
         return reducer.reduceNode(this)