code: @SuppressWarnings("unchecked")
      public <T extends WindupVertexFrame> T findSingletonVariable(String name)
      {
          Iterable<? extends WindupVertexFrame> frames = findVariable(name);
          if (null == frames)
          {
              throw new IllegalStateException("Variable not found: \"" + name + "\"");
          }
  
          Iterator<? extends WindupVertexFrame> iterator = frames.iterator();
          if (!iterator.hasNext())
          {
              return null;
          }
  
          WindupVertexFrame obj = iterator.next();
  
          if (iterator.hasNext())
          {
              throw new IllegalStateException("More than one frame present "
                          + "under presumed singleton variable: " + name);
          }
          return (T) obj;
      }
docs: Wrapper around {@link #findVariable(String)} which gives only one framed vertex, and checks if there is 0 or 1;
  throws otherwise.