code: public static void setCurrentPayload(Variables stack, String name, WindupVertexFrame frame)
                  throws IllegalArgumentException
      {
          Map<String, Iterable<? extends WindupVertexFrame>> vars = stack.peek();
  
          Iterable<? extends WindupVertexFrame> existingValue = vars.get(name);
          if (!(existingValue == null || existingValue instanceof IterationPayload))
          {
              throw new IllegalArgumentException("Variable \"" + name
                          + "\" has already been assigned and cannot be used as an " + Iteration.class.getSimpleName()
                          + " variable.");
          }
  
          vars.put(name, new IterationPayload<>(frame));
      }
docs: Set the current {@link Iteration} payload.