public String toTokenString() {
		CharStream input = this.token.getInputStream();
		GrammarASTAdaptor adaptor = new GrammarASTAdaptor(input);
		CommonTreeNodeStream nodes =
			new CommonTreeNodeStream(adaptor, this);
		StringBuilder buf = new StringBuilder();
		GrammarAST o = (GrammarAST)nodes.LT(1);
		int type = adaptor.getType(o);
		while ( type!=Token.EOF ) {
			buf.append(" ");
			buf.append(o.getText());
			nodes.consume();
			o = (GrammarAST)nodes.LT(1);
			type = adaptor.getType(o);
		}
		return buf.toString();
	}
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