function accumulator( x ) {
		if ( arguments.length === 0 ) {
			if ( N === 0 ) {
				return null;
			}
			return v;
		}
		N += 1;
		sum += ln( x );
		v = exp( sum/N );
		return v;
	}
If provided a value, the accumulator function returns an updated geometric mean. If not provided a value, the accumulator function returns the current geometric mean.

@private
@param {number} [x] - new value
@returns {(number|null)} geometric mean or null