function cdf( x, mu ) {
	return addon( x, mu );
}
Evaluates the cumulative distribution function (CDF) for a degenerate distribution with mean value `mu`.

@private
@param {number} x - input value
@param {number} mu - constant value of distribution
@returns {Probability} evaluated cumulative distribution function

@example
var y = cdf( 2.0, 3.0 );
// returns 0.0

@example
var y = cdf( 4.0, 3.0 );
// returns 1.0

@example
var y = cdf( 3.0, 3.0 );
// returns 1.0

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

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