code: @Override
      public void visitClassContext(ClassContext classContext) {
          try {
              stack = new OpcodeStack();
              graphicsRegs = new HashMap<Integer, Integer>(5);
              super.visitClassContext(classContext);
          } finally {
              stack = null;
              graphicsRegs = null;
          }
      }
docs: overrides the visitor to set up the opcode stack
  
  @param classContext
  the context object of the currently parsed class