A simple visitor, based upon the classic double dispatch method,
for walking GrammarAST trees resulting from parsing ANTLR grammars.
There is also the GrammarTreeVisitor.g tree grammar that looks for
subtree patterns and fires off high-level events as opposed to
"found node" events like this visitor does. Also, like all
visitors, the users of this interface are required to implement
the node visitation of the children. The GrammarTreeVisitor mechanism
fires events and the user is not required to do any walking code.
GrammarAST t = ...;
GrammarASTVisitor v = new ...;