code: def filenames(self):
          
          fns = set()
          for fn2var in self._type2filename2variable.values():
              fns.update(fn2var.keys())
          return sorted(fns)
docs: A list of all handled auxiliary file names.
  
          >>> from hydpy import dummies
          >>> dummies.v2af.filenames
          ['file1', 'file2']