function logpdf( x ) {
		if ( isnan( x ) ) {
			return NaN;
		}
		return ( x === mu ) ? PINF : NINF;
	}
Evaluates the natural logarithm of the probability density function (logPDF) of a degenerate distribution.

@private
@param {number} x - input value
@returns {number} natural logarithm of the probability density function

@example
var y = logpdf( 10.0 );
// returns <number>