protected static int getUniqueAlt(ATNConfigSet configs) {
		int alt = ATN.INVALID_ALT_NUMBER;
		for (ATNConfig c : configs) {
			if ( alt == ATN.INVALID_ALT_NUMBER ) {
				alt = c.alt; // found first alt
			}
			else if ( c.alt!=alt ) {
				return ATN.INVALID_ALT_NUMBER;
			}
		}
		return alt;
	}
Used for debugging in adaptivePredict around execATN but I cut
 it out for clarity now that alg. works well. We can leave this
 "dead" code for a bit.
runtime/Java/src/org/antlr/v4/runtime/atn/ParserATNSimulator.java