function incrmmda( W ) {
	var mean;
	var FLG;
	var f0;
	var a0;
	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 directional accuracy. If not provided input values, the accumulator function returns the current mean directional.
	*
	* @private
	* @param {number} [f] - forecast value
	* @param {number} [a] - actual value
	* @returns {(number|null)} mean directional accuracy or null
	*/
	function accumulator( f, a ) {
		var sf;
		var sa;
		if ( arguments.length === 0 ) {
			return mean();
		}
		if ( FLG === void 0 ) {
			FLG = true;
			f0 = f;
			a0 = a;
		}
		sf = signum( f-f0 );
		sa = signum( a-a0 );
		f0 = f;
		a0 = a;
		return mean( kroneckerDelta( sf, sa ) );
	}
}
Returns an accumulator function which incrementally computes a moving mean directional accuracy.

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

@example
var accumulator = incrmmda( 3 );

var m = accumulator();
// returns null

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

m = accumulator( 5.0, 2.0 );
// returns 0.5

m = accumulator( 3.0, 2.0 );
// returns ~0.33

m = accumulator( 4.0, 5.0 );
// returns ~0.33

m = accumulator();
// returns ~0.33