Rename nodes to more general names.

src/main/java/org/nwapw/abacus/tree/UnaryNode.java

@@ -0,0 +1,63 @@
+package org.nwapw.abacus.tree;
+
+public class UnaryNode extends TreeNode {
+
+    /**
+     * The operation this node will apply.
+     */
+    private String operation;
+    /**
+     * The tree node to apply the operation to.
+     */
+    private TreeNode applyTo;
+
+    /**
+     * Creates a new node with the given operation and no child.
+     *
+     * @param operation the operation for this node.
+     */
+    public UnaryNode(String operation) {
+        this(operation, null);
+    }
+
+    /**
+     * Creates a new node with the given operation and child.
+     *
+     * @param operation the operation for this node.
+     * @param applyTo   the node to apply the function to.
+     */
+    public UnaryNode(String operation, TreeNode applyTo) {
+        this.operation = operation;
+        this.applyTo = applyTo;
+    }
+
+    @Override
+    public <T> T reduce(Reducer<T> reducer) {
+        Object reducedChild = applyTo.reduce(reducer);
+        if (reducedChild == null) return null;
+        return reducer.reduceNode(this, reducedChild);
+    }
+
+    /**
+     * Gets the operation of this node.
+     *
+     * @return the operation this node performs.
+     */
+    public String getOperation() {
+        return operation;
+    }
+
+    /**
+     * Gets the node to which this node's operation applies.
+     *
+     * @return the tree node to which the operation will be applied.
+     */
+    public TreeNode getApplyTo() {
+        return applyTo;
+    }
+
+    @Override
+    public String toString() {
+        return "(" + (applyTo == null ? "null" : applyTo.toString()) + ")" + operation;
+    }
+}