Rewrite tree nodes in Kotlin. Documentation pending.

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

@@ -0,0 +1,31 @@
+package org.nwapw.abacus.tree
+
+data class FunctionNode(val function: String) : TreeNode() {
+
+    val children: MutableList<TreeNode> = mutableListOf()
+
+    override fun <T : Any> reduce(reducer: Reducer<T>): T? {
+        val children = Array<Any?>(children.size, { children[it].reduce(reducer) ?: return null; })
+        return reducer.reduceNode(this, *children)
+    }
+
+    override fun toString(): String {
+        val buffer = StringBuffer()
+        buffer.append(function)
+        buffer.append('(')
+        for (i in 0 until children.size) {
+            buffer.append(children[i].toString())
+            buffer.append(if (i == children.size - 1) ")" else ",")
+        }
+        return super.toString()
+    }
+
+    fun appendChild(node: TreeNode){
+        children.add(node)
+    }
+
+    fun prependChild(node: TreeNode){
+        children.add(0, node)
+    }
+
+}