code: public static void invalidArgIfNot(boolean tester, String msg, Object... args) {
          if (!tester) {
              throw invalidArg(msg, args);
          }
      }
docs: Throws out an {@link InvalidArgException} with error message specified
  when `tester` is `false`.
  
  @param tester
  when `false` then throws out the exception.
  @param msg
  the error message format pattern.
  @param args
  the error message format arguments.