function pdf( x, a, b ) {
	return addon( x, a, b );
}
Evaluates the probability density function (PDF) for an arcsine distribution with minimum support `a` and maximum support `b` at a value `x`.

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

@example
var y = pdf( 2.0, 0.0, 4.0 );
// returns ~0.159

@example
var y = pdf( 5.0, 0.0, 4.0 );
// returns 0.0

@example
var y = pdf( 0.25, 0.0, 1.0 );
// returns ~0.735

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

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

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

@example
var y = pdf( 2.0, 3.0, 1.0 );
// returns NaN