public boolean containsFast(T obj) {
		if (obj == null) {
			return false;
		}

		return get(obj) != null;
	}
Add {@code o} to set if not there; return existing value if already
there. This method performs the same operation as {@link #add} aside from
the return value.
runtime/Java/src/org/antlr/v4/runtime/misc/Array2DHashSet.java