public String getAltLabel() {
		List<? extends Tree> ancestors = this.getAncestors();
		if ( ancestors==null ) return null;
		for (int i=ancestors.size()-1; i>=0; i--) {
			GrammarAST p = (GrammarAST)ancestors.get(i);
			if ( p.getType()== ANTLRParser.ALT ) {
				AltAST a = (AltAST)p;
				if ( a.altLabel!=null ) return a.altLabel.getText();
				if ( a.leftRecursiveAltInfo!=null ) {
					return a.leftRecursiveAltInfo.altLabel;
				}
			}
		}
		return null;
	}
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