function logit( p ) {
	if ( isnan( p ) ) {
		return p;
	}
	if ( !isProbability( p ) ) {
		return NaN;
	}
	if ( p === 0.0 ) {
		return NINF;
	}
	if ( p === 1.0 ) {
		return PINF;
	}
	return ln( p / ( 1.0-p ) );
}
Evaluates the logit function.

@param {Probability} p - input value
@returns {number} function value

@example
var y = logit( 0.2 );
// returns ~-1.386

@example
var y = logit( 0.9 );
// returns ~2.197

@example
var y = logit( -4.0 );
// returns NaN

@example
var y = logit( 1.5 );
// returns NaN

@example
var y = logit( NaN );
// returns NaN