docs: @SuppressWarnings("unchecked")
      public <T extends WindupVertexFrame> Iterable<T> findVariableOfType(Class<T> type)
      {
          for (Map<String, Iterable<? extends WindupVertexFrame>> topOfStack : deque)
          {
              for (Iterable<? extends WindupVertexFrame> frames : topOfStack.values())
              {
                  boolean empty = true;
                  for (WindupVertexFrame frame : frames)
                  {
                      if (!type.isAssignableFrom(frame.getClass()))
                      {
                          break;
                      }
                      else
                      {
                          empty = false;
                      }
                  }
                  // now we know all the frames are of the chosen type
                  if (!empty)
                      return (Iterable<T>) frames;
              }
          }
          return null;
      }