function stdev( c ) {
	var k;
	if (
		isnan( c ) ||
		c <= 0.0
	) {
		return NaN;
	}
	k = ln( 1.0 + c );
	return sqrt( ( ( ( 2.0+c ) * k ) - ( 2.0*c ) ) / ( 2.0*c*k*k ) );
}
Returns the standard deviation of a Bradford distribution.

@param {PositiveNumber} c - shape parameter
@returns {PositiveNumber} standard deviation

@example
var v = stdev( 0.1 );
// returns ~0.289

@example
var v = stdev( 0.5 );
// returns ~0.288

@example
var v = stdev( 10.0 );
// returns ~0.276

@example
var v = stdev( 0.0 );
// returns NaN

@example
var v = stdev( -1.0 );
// returns NaN

@example
var v = stdev( NaN );
// returns NaN