function accumulator( f, a ) {
		if ( arguments.length === 0 ) {
			return mean();
		}
		return mean( atan( abs( (a-f)/a ) ) );
	}
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