function logpmf( x ) {
		if ( isnan( x ) ) {
			return NaN;
		}
		return ( x === mu ) ? 0.0 : NINF;
	}
Evaluates the natural logarithm of the probability mass function (PMF) for a degenerate distribution.

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

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