code: public static Predicate<Class<?>> classIsDescendantOf(Class<?> ancestor) {
          return candidate -> candidate != null && candidate != ancestor && ancestor
                  .isAssignableFrom(candidate);
      }
docs: Check if a candidate class is strictly a descendant of the specified class (not the
  specified class itself).
  
  @param ancestor the ancestor class to check for.
  @return the predicate.