public static int getUniqueAlt(Collection<BitSet> altsets) {
		BitSet all = getAlts(altsets);
		if ( all.cardinality()==1 ) return all.nextSetBit(0);
		return ATN.INVALID_ALT_NUMBER;
	}
Returns the unique alternative predicted by all alternative subsets in
{@code altsets}. If no such alternative exists, this method returns
{@link ATN#INVALID_ALT_NUMBER}.

@param altsets a collection of alternative subsets
runtime/Java/src/org/antlr/v4/runtime/atn/PredictionMode.java