function accumulator( x ) {
		if ( arguments.length === 0 || isnan( x ) ) {
			return stdev();
		}
		return stdev( x );
	}
If provided a value, the accumulator function returns an updated corrected sample standard deviation. If not provided a value, the accumulator function returns the current corrected sample standard deviation.

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