protected RuleTagToken getRuleTagToken(ParseTree t) {
		if ( t instanceof RuleNode ) {
			RuleNode r = (RuleNode)t;
			if ( r.getChildCount()==1 && r.getChild(0) instanceof TerminalNode ) {
				TerminalNode c = (TerminalNode)r.getChild(0);
				if ( c.getSymbol() instanceof RuleTagToken ) {
//					System.out.println("rule tag subtree "+t.toStringTree(parser));
					return (RuleTagToken)c.getSymbol();
				}
			}
		}
		return null;
	}
Is {@code t} {@code (expr <expr>)} subtree?
runtime/Java/src/org/antlr/v4/runtime/tree/pattern/ParseTreePatternMatcher.java