function cdf( x, k, lambda ) {
	return addon( x, k, lambda );
}
Evaluates the cumulative distribution function (CDF) for a Weibull distribution with shape parameter `k` and scale parameter `lambda` at a value `x`.

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

@example
var y = cdf( 2.0, 1.0, 1.0 );
// returns ~0.865

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

@example
var y = cdf( +Infinity, 4.0, 2.0 );
// returns 1.0

@example
var y = cdf( -Infinity, 4.0, 2.0 );
// returns 0.0

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

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

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

@example
var y = cdf( 2.0, 0.0, -1.0 );
// returns NaN