code: def set_substitution(self, what, rep):
          
          if rep is None:
              # Unset the variable.
              if what in self._subs:
                  del self._subs[what]
          self._subs[what] = rep
docs: Set a substitution.
  
          Equivalent to ``! sub`` in RiveScript code.
  
          :param str what: The original text to replace.
          :param str rep: The text to replace it with.
              Set this to ``None`` to delete the substitution.