function accumulator( x, y ) {
		var r;
		if ( arguments.length === 0 ) {
			return sum();
		}
		r = y - x;
		return sum( r*r );
	}
If provided input values, the accumulator function returns an updated residual sum of squares. If not provided input values, the accumulator function returns the current residual sum of squares.

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