code: public final void addClass(final SgClass clasz) {
          if (clasz == null) {
              throw new IllegalArgumentException("The argument 'clasz' cannot be null!");
          }
          if (!classes.contains(clasz)) {
              classes.add(clasz);
          }
      }
docs: Adds an inner to this class. Does nothing if the class is already in the
  list of inner classes.
  
  @param clasz
  Inner class to add - Cannot be null.