function stdev( N, K, n ) {
	return addon( N, K, n );
}
Returns the standard deviation of a hypergeometric distribution.

@private
@param {NonNegativeInteger} N - population size
@param {NonNegativeInteger} K - subpopulation size
@param {NonNegativeInteger} n - number of draws
@returns {NonNegativeNumber} standard deviation

@example
var v = stdev( 16, 11, 4 );
// returns ~0.829

@example
var v = stdev( 2, 1, 1 );
// returns 0.5

@example
var v = stdev( 10, 5, 12 );
// returns NaN

@example
var v = stdev( -2, 4, 2 );
// returns NaN

@example
var v = stdev( 20, -2, 10 );
// returns NaN

@example
var v = stdev( 40, 20, -2 );
// returns NaN