function cdf( x ) {
		var xc;
		if ( isnan( x ) ) {
			return NaN;
		}
		xc = x - mu;
		return 0.5 * erfc( -xc / denom );
	}
Evaluates the cumulative distribution function (CDF) for a Normal distribution.

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

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