class_docstr: A dictionary-like class that can be used to assign unique IDs to
  names (say, vertex names).
  
  Usage:
  
  >>> gen = UniqueIdGenerator()
  >>> gen["A"]
  0
  >>> gen["B"]
  1
  >>> gen["C"]
  2
  >>> gen["A"]      # Retrieving already existing ID
  0
  >>> gen.add("D")  # Synonym of gen["D"]
  3
  >>> len(gen)      # Number of already used IDs
  4
  >>> "C" in gen
  True
  >>> "E" in gen
  False
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docstr: Returns the values stored so far. If the generator generates items
  according to the standard sorting order, the values returned will be
  exactly in the order they were added. This holds for integer IDs for
  instance (but for many other ID generators as well).
code: def values(self):
      """Returns the values stored so far. If the generator generates items
      according to the standard sorting order, the values returned will be
      exactly in the order they were added. This holds for integer IDs for
      instance (but for many other ID generators as well)."""
      return sorted(self._ids.keys(), key=self._ids.__getitem__)
  
label_desc: Use image node_id 7 for calling the UniqueIdGenerator obj's underlying member method code with example usage: obj.values() and returns: sorted