protected static void combineCommonParents(PredictionContext[] parents) {
		Map<PredictionContext, PredictionContext> uniqueParents =
			new HashMap<PredictionContext, PredictionContext>();

		for (int p = 0; p < parents.length; p++) {
			PredictionContext parent = parents[p];
			if ( !uniqueParents.containsKey(parent) ) { // don't replace
				uniqueParents.put(parent, parent);
			}
		}

		for (int p = 0; p < parents.length; p++) {
			parents[p] = uniqueParents.get(parents[p]);
		}
	}
Make pass over all <em>M</em> {@code parents}; merge any {@code equals()}
ones.
runtime/Java/src/org/antlr/v4/runtime/atn/PredictionContext.java