function factory( a, b ) {
	if (
		isnan( a ) ||
		isnan( b ) ||
		a >= b
	) {
		return constantFunction( NaN );
	}
	return cdf;

	/**
	* Evaluates the cumulative distribution function (CDF) for an arcsine distribution.
	*
	* @private
	* @param {number} x - input value
	* @returns {Probability} evaluated CDF
	*
	* @example
	* var y = cdf( 2.0 );
	* // returns <number>
	*/
	function cdf( x ) {
		if ( isnan( x ) ) {
			return NaN;
		}
		if ( x < a ) {
			return 0.0;
		}
		if ( x >= b ) {
			return 1.0;
		}
		return TWO_OVER_PI * asin( sqrt( ( x-a ) / ( b-a ) ) );
	}
}
Returns a function for evaluating the cumulative distribution function (CDF) for an arcsine distribution with minimum support `a` and maximum support `b`.

@param {number} a - minimum support
@param {number} b - maximum support
@returns {Function} CDF

@example
var cdf = factory( 0.0, 10.0 );
var y = cdf( 0.5 );
// returns ~0.144

y = cdf( 8.0 );
// returns ~0.705