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

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

@returns {Function} accumulator function

@example
var accumulator = incrmaape();

var m = accumulator();
// returns null

m = accumulator( 2.0, 3.0 );
// returns ~0.3218

m = accumulator( 5.0, 2.0 );
// returns ~0.6523

m = accumulator();
// returns ~0.6523