code: public String checkSanity() {
          for (CronTab tab : tabs) {
              String s = tab.checkSanity();
              if(s!=null)     return s;
          }
          return null;
      }
docs: Checks if this crontab entry looks reasonable,
  and if not, return an warning message.
  
  <p>
  The point of this method is to catch syntactically correct
  but semantically suspicious combinations, like
  "* 0 * * *"