function logcdf( x, mu ) {
	return addon( x, mu );
}
Evaluates the natural logarithm of the cumulative distribution function (logCDF) for a degenerate distribution with mean `mu`.

@private
@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