does not exist at <code>x</code>,
 then an appropriate replacement should be returned, e.g. <code>Double.POSITIVE_INFINITY</code>,
 <code>Double.NaN</code>, or the limit inferior or limit superior of the difference quotient. Note
 that due to the floating point precision and under/overflow issues, this method will for some
 distributions be more precise and faster than computing the logarithm of
 <a href="../../../../../org/apache/commons/math3/distribution/RealDistribution.html#density(double)"><code>RealD