code: def add_subkey(self, subkey):
          
          if self.key.lower() == 'atoms' or not self.has_subkey(subkey):
              self.subkeys.append(subkey)
docs: Add a new subkey to this key.
  
          Parameters
          ----------
          subkey : AdfKey
              A new subkey.
  
          Notes
          -----
          Duplicate check will not be performed if this is an 'Atoms' block.