Fix toString errors failing unit tests.

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

@@ -28,7 +28,7 @@ data class FunctionNode(val function: String) : TreeNode() {
             buffer.append(children[i].toString())
             buffer.append(if (i == children.size - 1) ")" else ",")
         }
-        return super.toString()
+        return buffer.toString()
     }
 
     /**

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

@@ -16,4 +16,8 @@ data class UnaryNode(val operation: String, val applyTo: TreeNode? = null) : Tre
         return reducer.reduceNode(this, reducedChild)
     }
 
+    override fun toString(): String {
+        return "(" + (applyTo?.toString() ?: "null") + ")" + operation
+    }
+
 }