function accumulator( x, y ) {
		if ( arguments.length === 0 ) {
			return mean();
		}
		return mean( y-x );
	}
If provided input values, the accumulator function returns an updated mean error. If not provided input values, the accumulator function returns the current mean error.

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