public GrammarAST dupTree() {
		GrammarAST t = this;
		CharStream input = this.token.getInputStream();
		GrammarASTAdaptor adaptor = new GrammarASTAdaptor(input);
		return (GrammarAST)adaptor.dupTree(t);
	}
Walk ancestors of this node until we find ALT with
 alt!=null or leftRecursiveAltInfo!=null. Then grab label if any.
 If not a rule element, just returns null.
tool/src/org/antlr/v4/tool/ast/GrammarAST.java