function accumulator( x ) {
		if ( arguments.length === 0 ) {
			return ( s === void 0 ) ? null : s;
		}
		if ( s === void 0 ) {
			m = x;
			s2 = 0.0;
		} else {
			r = x - m;
			incr = alpha * r;
			m += incr;
			s2 = c * ( s2+(r*incr) );
		}
		s = sqrt( s2 );
		return s;
	}
If provided a value, the accumulator function returns an updated standard deviation. If not provided a value, the accumulator function returns the current standard deviation.

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