function skewness( p ) {
	if (
		isnan( p ) ||
		p <= 0.0 ||
		p >= 1.0
	) {
		return NaN;
	}
	return ( 2.0-p ) / sqrt( 1.0-p );
}
Returns the skewness of a geometric distribution.

@param {Probability} p - success probability
@returns {PositiveNumber} skewness

@example
var v = skewness( 0.1 );
// returns ~2.003

@example
var v = skewness( 0.5 );
// returns ~2.121

@example
var v = skewness( 1.1 );
// returns NaN

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