docs: def gen_variable_completions(self, word, env):
          """Generate a sequence of possible variable completions for ``word``.
  
          :param word: the word to complete
          :param env: the environment
          """
          # ignore the first character, which is a dollar sign
          var = word[1:]
          for k in env:
              if k.startswith(var):
                  yield "$" + k