function incrmmae( W ) {
	var mean;
	if ( !isPositiveInteger( W ) ) {
		throw new TypeError( format( 'invalid argument. Must provide a positive integer. Value: `%s`.', W ) );
	}
	mean = incrmmean( W );
	return accumulator;

	/**
	* If provided input values, the accumulator function returns an updated mean absolute error. If not provided input values, the accumulator function returns the current mean absolute error.
	*
	* @private
	* @param {number} [x] - input value
	* @param {number} [y] - input value
	* @returns {(number|null)} mean absolute error or null
	*/
	function accumulator( x, y ) {
		if ( arguments.length === 0 ) {
			return mean();
		}
		return mean( abs( y-x ) );
	}
}
Returns an accumulator function which incrementally computes a moving mean absolute error.

@param {PositiveInteger} W - window size
@throws {TypeError} must provide a positive integer
@returns {Function} accumulator function

@example
var accumulator = incrmmae( 3 );

var m = accumulator();
// returns null

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

m = accumulator( -5.0, 2.0 );
// returns 4.0

m = accumulator( 3.0, 2.0 );
// returns 3.0

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

m = accumulator();
// returns 5.0