function cdf( x, mu, s ) {
	var z;
	if (
		isnan( x ) ||
		isnan( mu ) ||
		isnan( s ) ||
		s < 0.0
	) {
		return NaN;
	}
	if ( s === 0.0 ) {
		return ( x < mu ) ? 0.0 : 1.0;
	}
	z = ( x - mu ) / s;
	return 1.0 / ( 1.0 + exp( -z ) );
}
Evaluates the cumulative distribution function (CDF) for a logistic distribution with location parameter `mu` and scale parameter `s` at a value `x`.

@param {number} x - input value
@param {number} mu - location parameter
@param {NonNegativeNumber} s - scale parameter
@returns {Probability} evaluated CDF

@example
var y = cdf( 2.0, 0.0, 1.0 );
// returns ~0.881

@example
var y = cdf( 5.0, 10.0, 3.0 );
// returns ~0.159

@example
var y = cdf( 2.0, 0.0, NaN );
// returns NaN

@example
var y = cdf( 2.0, NaN, 1.0 );
// returns NaN

@example
var y = cdf( NaN, 0.0, 1.0 );
// returns NaN