code: public MultipleRuleContext<DPO, RI, RO> check(Rule<RI, RO> rule) {
          if (rule != null) {
              addedRules.add(rule);
          }
  
          // Stay in the same context and re-use the same instance because no type has changed
          return this;
      }
docs: Adds the specified rule to the validator under construction.
  
  @param rule Rule to be added.
  
  @return Context allowing further construction of the validator using the DSL.