/** What does a tree look like?  ANTLR has a number of support classes
 *  such as CommonTreeNodeStream that work on these kinds of trees.  You
 *  don't have to make your trees implement this interface, but if you do,
 *  you'll be able to use more support code.
 *
 *  NOTE: When constructing trees, ANTLR can build any kind of tree; it can
 *  even use Token objects as trees if you add a child list to your tokens.
 *
 *  This is a tree node without any payload; just navigation and factory stuff.
 */
org.antlr.runtime.tree.Tree = function(){};