Add comments to the newly created Kotlin implementations.

src/main/kotlin/org/nwapw/abacus/tree/BinaryNode.kt

@@ -1,5 +1,16 @@
 package org.nwapw.abacus.tree
 
+/**
+ * A tree node that holds a binary operation.
+ *
+ * This node represents any binary operation, such as binary infix or binary postfix. The only
+ * currently implemented into Abacus is binary infix, but that has more to do with the parser than
+ * this class, which doesn't care about the order that its operation and nodes were found in text.
+ *
+ * @param operation the operation this node performs on its children.
+ * @param left the left node.
+ * @param right the right node.
+ */
 data class BinaryNode(val operation: String, val left: TreeNode? = null, val right: TreeNode?) : TreeNode() {
 
     override fun <T : Any> reduce(reducer: Reducer<T>): T? {

src/main/kotlin/org/nwapw/abacus/tree/FunctionNode.kt

@@ -1,7 +1,18 @@
 package org.nwapw.abacus.tree
 
+/**
+ * A tree node that holds a function call.
+ *
+ * The function call node can hold any number of children, and passes the to the appropriate reducer,
+ * but that is its sole purpose.
+ *
+ * @param function the function string.
+ */
 data class FunctionNode(val function: String) : TreeNode() {
 
+    /**
+     * List of function parameters added to this node.
+     */
     val children: MutableList<TreeNode> = mutableListOf()
 
     override fun <T : Any> reduce(reducer: Reducer<T>): T? {
@@ -20,10 +31,20 @@ data class FunctionNode(val function: String) : TreeNode() {
         return super.toString()
     }
 
+    /**
+     * Appends a child to this node's list of children.
+     *
+     * @node the node to append.
+     */
     fun appendChild(node: TreeNode){
         children.add(node)
     }
 
+    /**
+     * Prepends a child to this node's list of children.
+     *
+     * @node the node to prepend.
+     */
     fun prependChild(node: TreeNode){
         children.add(0, node)
     }

src/main/kotlin/org/nwapw/abacus/tree/NumberNode.kt

@@ -2,6 +2,14 @@ package org.nwapw.abacus.tree
 
 import org.nwapw.abacus.number.NumberInterface
 
+/**
+ * A tree node that holds a single number value.
+ *
+ * This is a tree node that holds a single NumberInterface, which represents any number,
+ * and is not defined during compile time.
+ *
+ * @number the number value of this node.
+ */
 data class NumberNode(val number: NumberInterface) : TreeNode() {
 
     override fun <T : Any> reduce(reducer: Reducer<T>): T? {

src/main/kotlin/org/nwapw/abacus/tree/TreeNode.kt

@@ -1,5 +1,8 @@
 package org.nwapw.abacus.tree
 
+/**
+ * A tree node.
+ */
 abstract class TreeNode {
 
     abstract fun <T: Any> reduce(reducer: Reducer<T>) : T?

src/main/kotlin/org/nwapw/abacus/tree/UnaryNode.kt

@@ -1,5 +1,14 @@
 package org.nwapw.abacus.tree
 
+/**
+ * A tree node that holds a unary operation.
+ *
+ * This node holds a single operator applied to a single parameter, and does not care
+ * whether the operation was found before or after the parameter in the text.
+ *
+ * @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() {
 
     override fun <T : Any> reduce(reducer: Reducer<T>): T? {