docs: def execute_once(self, string):
          """Execute only one rule."""
          for rule in self.rules:
              if rule[0] in string:
                  pos = string.find(rule[0])
                  self.last_rule = rule
                  return string[:pos] + rule[1] + string[pos+len(rule[0]):]
          self.last_rule = None
          return string