docs: def _capture_variable(iterator, parameters):
      """
      return the replacement string.
      this assumes the preceeding {{ has already been
      popped off.
      """
      key = ""
      next_c = next(iterator)
      while next_c != "}":
          key += next_c
          next_c = next(iterator)
      # remove the final "}"
      next(iterator)
      return parameters[key]