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 number of items.
code: def __len__(self):
      """Returns the number of items."""
      return len(self._ids)
  
label_desc: Use image node_id 5 for calling the UniqueIdGenerator obj's underlying member method code with example usage: obj.__len__() and returns: len