function factory( mu ) {
	if ( isnan( mu ) ) {
		return constantFunction( NaN );
	}
	return cdf;

	/**
	* Evaluates the cumulative distribution function (CDF) of a degenerate distribution.
	*
	* @private
	* @param {number} x - input value
	* @returns {Probability} evaluated cumulative distribution function
	*
	* @example
	* var y = cdf( 10.0 );
	* // returns <number>
	*/
	function cdf( x ) {
		if ( isnan( x ) ) {
			return NaN;
		}
		return (x < mu) ? 0.0 : 1.0;
	}
}
Returns a function for evaluating the cumulative distribution function (CDF) of a degenerate distribution centered at a provided mean value.

@param {number} mu - constant value of distribution
@returns {Function} function to evaluate the cumulative distribution function

@example
var cdf = factory( 5.0 );

var y = cdf( 3.0 );
// returns 0.0

y = cdf( 6.0 );
// returns 1.0

y = cdf( NaN );
// returns NaN