code: def get_filename(self, variable):
          
          fn2var = self._type2filename2variable.get(type(variable), {})
          for (fn_, var) in fn2var.items():
              if var == variable:
                  return fn_
          return None
docs: Return the auxiliary file name the given variable is allocated
          to or |None| if the given variable is not allocated to any
          auxiliary file name.
  
          >>> from hydpy import dummies
          >>> eqb = dummies.v2af.eqb[0]
          >>> dummies.v2af.get_filename(eqb)
          'file1'
          >>> eqb += 500.0
          >>> dummies.v2af.get_filename(eqb)