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

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