@Override
	protected void enterBlockSet(GrammarAST tree) {
		frequencies.push(new FrequencySet<String>());
		minFrequencies.push(new FrequencySet<String>());
	}
Generate a frequency set as the union of two input sets, with the
values clipped to a specified maximum value. If an element is
contained in both sets, the value for the output, prior to clipping,
will be the sum of the two input values.

@param a The first set.
@param b The second set.
@param clip The maximum value to allow for any output.
@return The sum of the two sets, with the individual elements clipped
to the maximum value given by {@code clip}.
tool/src/org/antlr/v4/codegen/model/ElementFrequenciesVisitor.java