function accumulator( x, y ) {
		if ( arguments.length === 0 ) {
			return sum();
		}
		return sum( x*y );
	}
If provided a value, the accumulator function returns an updated sum. If not provided a value, the accumulator function returns the current sum.

@private
@param {number} [x] - input value
@param {number} [y] - input value
@returns {(number|null)} sum or null