function cdf( x, lambda ) {
	if ( isnan( x ) || isnan( lambda ) || lambda < 0.0 ) {
		return NaN;
	}
	if ( x < 0.0 ) {
		return 0.0;
	}
	if ( lambda === 0.0 ) {
		return 1.0;
	}
	if ( x === PINF ) {
		return 1.0;
	}
	return gammainc( lambda, floor( x ) + 1.0, true, true );
}
Evaluates the cumulative distribution function (CDF) for a Poisson distribution with mean parameter `lambda` at a value `x`.

@param {number} x - input value
@param {NonNegativeNumber} lambda - mean parameter
@returns {Probability} evaluated CDF

@example
var y = cdf( 2.0, 0.5 );
// returns ~0.986

@example
var y = cdf( 2.0, 10.0 );
// returns ~0.003

@example
var y = cdf( -1.0, 4.0 );
// returns 0.0

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

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

@example
// Negative mean parameter:
var y = cdf( 2.0, -1.0 );
// returns NaN