function logpdf( x ) {
		if ( isnan( x ) ) {
			return NaN;
		}
		if ( x < a || x > b ) {
			return NINF;
		}
		return -( LN_PI + ( ln( ( x-a ) * ( b-x ) ) / 2.0 ) );
	}
Evaluates the logarithm of the probability density function (PDF) for an arcsine distribution.

@private
@param {number} x - input value
@returns {number} evaluated logPDF

@example
var y = logpdf( 2.0 );
// returns <number>