@Override
	public void clear() {
		n = 0;
		buckets = createBuckets(this.initialCapacity);
		threshold = (int)Math.floor(this.initialCapacity * LOAD_FACTOR);
	}
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