function cdf( x ) {
		if ( isnan( x ) ) {
			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.

@private
@param {number} x - input value
@returns {Probability} evaluated CDF

@example
var y = cdf( 2.0 );
// returns <number>