function stdev( v ) {
	return addon( v );
}
Returns the standard deviation of a Student's t distribution.

@private
@param {PositiveNumber} v - degrees of freedom
@returns {PositiveNumber} standard deviation

@example
var v = stdev( 9.0 );
// returns ~1.134

@example
var v = stdev( 2.0 );
// returns Infinity

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

@example
var v = stdev( -0.2 );
// returns NaN

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