code: def make_inv_cts(cts):
      
      inv_ct = {}
      for old_k, old_v in cts.items():
          if old_v not in inv_ct.keys():
              inv_ct.update({old_v : old_k})
      return inv_ct
docs: cts is e.g. {"Germany" : "DEU"}. inv_cts is the inverse: {"DEU" : "Germany"}