function cdf( x, a, b ) {
	if (
		isnan( x ) ||
		isnan( a ) ||
		isnan( b ) ||
		a >= b
	) {
		return NaN;
	}
	if ( x < a ) {
		return 0.0;
	}
	if ( x >= b ) {
		return 1.0;
	}
	return TWO_OVER_PI * asin( sqrt( ( x-a ) / ( b-a ) ) );
}
Evaluates the cumulative distribution function (CDF) for an arcsine distribution with minimum support `a` and maximum support `b` at a value `x`.

@param {number} x - input value
@param {number} a - minimum support
@param {number} b - maximum support
@returns {Probability} evaluated CDF

@example
var y = cdf( 9.0, 0.0, 10.0 );
// returns ~0.795

@example
var y = cdf( 0.5, 0.0, 2.0 );
// returns ~0.333

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

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

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

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

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

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