function cdf( x ) {
		if ( isnan( x ) ) {
			return NaN;
		}
		if ( x <= 0.0 ) {
			return 0.0;
		}
		if ( x === PINF ) {
			return 1.0;
		}
		if ( d1 * x > d2 ) {
			return betainc( (d1*x)/(d2+(d1*x)), d1/2.0, d2/2.0, true, false );
		}
		return betainc( d2/(d2+(d1*x)), d2/2.0, d1/2.0, true, true );
	}
Evaluates the cumulative distribution function (CDF) for an F distribution.

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

@example
var y = cdf( 10.0 );
// returns <number>