protected final int getBucket(T o) {
		int hash = comparator.hashCode(o);
		int b = hash & (buckets.length-1); // assumes len is power of 2
		return b;
	}
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