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 ) );
	}
If provided a value, the accumulator function returns an updated mean directional accuracy. If not provided a value, the accumulator function returns the current mean directional accuracy.

@private
@param {number} [f] - forecast value
@param {number} [a] - actual value
@returns {(number|null)} mean directional accuracy or null