FrequencySet<String> getMinFrequencies() {
		assert minFrequencies.size() == 1;
		assert minFrequencies.peek() != SENTINEL;
		assert SENTINEL.isEmpty();

		return minFrequencies.peek();
	}
This special value means "no set", and is used by {@link #minFrequencies}
to ensure that {@link #combineMin} doesn't merge an empty set (all zeros)
with the results of the first alternative.
tool/src/org/antlr/v4/codegen/model/ElementFrequenciesVisitor.java