code: public void setVariable(String name, Iterable<? extends WindupVertexFrame> frames)
      {
          Map<String, Iterable<? extends WindupVertexFrame>> frame = peek();
          if (!Iteration.DEFAULT_VARIABLE_LIST_STRING.equals(name) && findVariable(name) != null)
          {
              throw new IllegalArgumentException("Variable \"" + name
                          + "\" has already been assigned and cannot be reassigned");
          }
  
          frame.put(name, frames);
      }
docs: Set a variable in the top variables layer to given "collection" of the vertex frames. Can't be reassigned -
  throws on attempt to reassign.