@Override
	public boolean equals(Object o) {
		if (o == this) return true;
		if ( !(o instanceof Array2DHashSet) ) return false;
		Array2DHashSet<?> other = (Array2DHashSet<?>)o;
		if ( other.size() != size() ) return false;
		boolean same = this.containsAll(other);
		return same;
	}
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