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With inspiration from other libraries
There is a newer version: 2.12.15
/*
 [The "BSD license"]
 Copyright (c) 2005-2009 Terence Parr
 All rights reserved.

 Redistribution and use in source and binary forms, with or without
 modification, are permitted provided that the following conditions
 are met:
 1. Redistributions of source code must retain the above copyright
     notice, this list of conditions and the following disclaimer.
 2. Redistributions in binary form must reproduce the above copyright
     notice, this list of conditions and the following disclaimer in the
     documentation and/or other materials provided with the distribution.
 3. The name of the author may not be used to endorse or promote products
     derived from this software without specific prior written permission.

 THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
 IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
 IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
 INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
 NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

package org.antlr.runtime.tree;

/** Do a depth first walk of a tree, applying pre() and post() actions
 *  as we discover and finish nodes.
 */
public class TreeVisitor {
    protected TreeAdaptor adaptor;
    
    public TreeVisitor(TreeAdaptor adaptor) {
        this.adaptor = adaptor;
    }
    public TreeVisitor() { this(new CommonTreeAdaptor()); }
    
    /** Visit every node in tree t and trigger an action for each node
     *  before/after having visited all of its children.
     *  Execute both actions even if t has no children.
     *  If a child visit yields a new child, it can update its
     *  parent's child list or just return the new child.  The
     *  child update code works even if the child visit alters its parent
     *  and returns the new tree.
     *
     *  Return result of applying post action to this node.
     */
    public Object visit(Object t, TreeVisitorAction action) {
        // System.out.println("visit "+((Tree)t).toStringTree());
        boolean isNil = adaptor.isNil(t);
        if ( action!=null && !isNil ) {
            t = action.pre(t); // if rewritten, walk children of new t
        }
        for (int i=0; i