code: public Iterable<? extends WindupVertexFrame> findVariable(String name, int maxDepth)
      {
          int currentDepth = 0;
          Iterable<? extends WindupVertexFrame> result = null;
          for (Map<String, Iterable<? extends WindupVertexFrame>> frame : deque)
          {
              result = frame.get(name);
              if (result != null)
              {
                  break;
              }
              currentDepth++;
              if (currentDepth >= maxDepth)
                  break;
          }
          return result;
      }
docs: Searches the variables layers, top to bottom, for given name, and returns if found; null otherwise.
  
  If maxDepth is set to {@link Variables#SEARCH_ALL_LAYERS}, then search all layers.