x: a list with components x and y. The vector y are the relative density estimates at the values in the vector x. The latter are in the unit interval.
xo: a list with components x and y like the first argument. If given the relative entropy (or Kullback-Liebler divergence) of the first to the second argument is calculated.
Returns
If xo is given, the relative entropy (or Kullback-Liebler divergence) of the first to the second argument is calculated. Otherwise the entropy of the distribution specified by x.