function cdf( x, lambda ) {
	if ( isnan( x ) || isnan( lambda ) || lambda <= 0.0 ) {
		return NaN;
	}
	if ( x < 0.0 ) {
		return 0.0;
	}
	if ( x === PINF ) {
		return 1.0;
	}
	return -expm1( -lambda * ( floor(x)+1.0 ) );
}
Evaluates the cumulative distribution function (CDF) for a Planck distribution with shape parameter `lambda` at a value `x`.

@param {number} x - input value
@param {PositiveNumber} lambda - shape parameter
@returns {Probability} evaluated CDF

@example
var y = cdf( 2.0, 0.5 );
// returns ~0.7769

@example
var y = cdf( 2.0, 1.5 );
// returns ~0.9889

@example
var y = cdf( -1.0, 0.5 );
// returns 0.0

@example
var y = cdf( NaN, 0.5 );
// returns NaN

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

@example
// Invalid shape parameter
var y = cdf( 2.0, -1.4 );
// returns NaN