function incrrmse() {
	var mean = incrmean();
	return accumulator;

	/**
	* 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
	*/
	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 ) );
	}
}
Returns an accumulator function which incrementally computes the root mean squared error.

@returns {Function} accumulator function

@example
var accumulator = incrrmse();

var r = accumulator();
// returns null

r = accumulator( 2.0, 3.0 );
// returns 1.0

r = accumulator( -5.0, 2.0 );
// returns 5.0

r = accumulator();
// returns 5.0