function logpdf( x, a, b ) {
	return addon( x, a, b );
}
Evaluates the logarithm of 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 {Probability} evaluated logPDF

@example
var y = logpdf( 2.0, 0.0, 4.0 );
// returns ~-1.838

@example
var y = logpdf( 5.0, 0.0, 4.0 );
// returns -Infinity

@example
var y = logpdf( 0.25, 0.0, 1.0 );
// returns ~-0.308

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

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

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

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