code: public T getById(String id) throws NRE {
          T rule = rules.get(id);
  
          if (null == rule) {
              throw newNoRuleException(id);
          }
  
          return rule;
      }
docs: Returns a rule of type {@code T} by its id.
  
  @return The rule with the requested id.
  
  @throws <NRE> if the requested rule cannot be found.