public void add(T key) {
		MutableInt value = get(key);
		if (value == null) {
			value = new MutableInt(1);
			put(key, value);
		}
		else {
			value.v++;
		}
	}
Count how many of each key we have; not thread safe
tool/src/org/antlr/v4/misc/FrequencySet.java