code: protected static <U> U checkedCall(Callable<U> callable) {
          try {
              return callable.call();
          } catch (Throwable ex) {
              if (ex instanceof RuntimeException) {
                  throw (RuntimeException)ex;
              }
              if (ex instanceof Error) {
                  throw (Error)ex;
              }
              throw new RuntimeException(ex);
          }
      }
docs: Calls the given callable and rethrows its exception
  (as RuntimeException if necessary).
  @param <U> the value type
  @param callable the callable to call
  @return the value returned by the callable