anchor.diff:  
   This function will return `NaN` if `inform_shannon` returns `NaN` when applied to any of the
   distribution arguments.
  +
  +[horizontal]
  +Header::
  +    `inform/shannon.h`
   ****
   
   ****
  
positive.code: `!inform_dist_is_valid(dist)`.
  
  See <<Shannon1948>> for more details.
  ****
  
  ****
  
negative.code:         double base);
  ----
  Compute the point-wise mutual information of an combination of events
  
  [horizontal]
  Header::
      `inform/shannon.h`
  ****
  
  ****
  