function cdf( x, d1, d2 ) {
	if (
		isnan( x ) ||
		isnan( d1 ) ||
		isnan( d2 ) ||
		d1 <= 0.0 ||
		d2 <= 0.0
	) {
		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 with numerator degrees of freedom `d1` and denominator degrees of freedom `d2` at a value `x`.

@param {number} x - input value
@param {PositiveNumber} d1 - numerator degrees of freedom
@param {PositiveNumber} d2 - denominator degrees of freedom
@returns {Probability} evaluated CDF

@example
var y = cdf( 2.0, 1.0, 1.0 );
// returns ~0.608

@example
var y = cdf( 2.0, 8.0, 4.0 );
// returns ~0.737

@example
var y = cdf( -1.0, 2.0, 2.0 );
// returns 0.0

@example
var y = cdf( +Infinity, 4.0, 2.0 );
// returns 1.0

@example
var y = cdf( -Infinity, 4.0, 2.0 );
// returns 0.0

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

@example
var y = cdf( 0.0, NaN, 1.0 );
// returns NaN

@example
var y = cdf( 0.0, 1.0, NaN );
// returns NaN

@example
var y = cdf( 2.0, 1.0, -1.0 );
// returns NaN

@example
var y = cdf( 2.0, -1.0, 1.0 );
// returns NaN