function accumulator( x, w ) {
		if ( arguments.length === 0 ) {
			if ( FLG === void 0 ) {
				return null;
			}
			return mu;
		}
		FLG = true;
		wsum += w;
		mu += ( w/wsum ) * ( x-mu );
		return mu;
	}
If provided arguments, the accumulator function returns an updated weighted mean. If not provided arguments, the accumulator function returns the current weighted mean.

@private
@param {number} [x] - value
@param {number} [w] - weight
@returns {(number|null)} weighted mean or null