code: def get_extra_imts(self, imts):
          
          extra_imts = set()
          for taxonomy in self.taxonomies:
              for (lt, kind), rf in self[taxonomy].risk_functions.items():
                  if rf.imt not in imts:
                      extra_imts.add(rf.imt)
          return extra_imts
docs: Returns the extra IMTs in the risk functions, i.e. the ones not in
          the `imts` set (the set of IMTs for which there is hazard).