function logcdf( x, mu ) {
	if ( isnan( x ) || isnan( mu ) ) {
		return NaN;
	}
	return ( x < mu ) ? NINF : 0.0;
}
Evaluates the natural logarithm of the cumulative distribution function (logCDF) for a degenerate distribution with mean `mu`.

@param {number} x - input value
@param {number} mu - constant value of distribution
@returns {number} natural logarithm of cumulative distribution function

@example
var y = logcdf( 2.0, 3.0 );
// returns -Infinity

@example
var y = logcdf( 4.0, 3.0 );
// returns 0.0

@example
var y = logcdf( 3.0, 3.0 );
// returns 0.0

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

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