function cdf( x ) {
		if ( isnan( x ) ) {
			return NaN;
		}
		if ( x < a ) {
			return 0.0;
		}
		if ( x >= b ) {
			return 1.0;
		}
		return ( x - a ) / ( b - a );
	}
Evaluates the cumulative distribution function (CDF) for a uniform distribution.

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

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