@SuppressWarnings("unchecked")
   private static void addToVariablesToRemove(InternalThreadLocalMap threadLocalMap, FastThreadLocal<?> variable) {
       Object v = threadLocalMap.indexedVariable(variablesToRemoveIndex);
       Set<FastThreadLocal<?>> variablesToRemove;
       if (v == InternalThreadLocalMap.UNSET || v == null) {
           variablesToRemove = Collections.newSetFromMap(new IdentityHashMap<FastThreadLocal<?>, Boolean>());
           threadLocalMap.setIndexedVariable(variablesToRemoveIndex, variablesToRemove);
       } else {
           variablesToRemove = (Set<FastThreadLocal<?>>) v;
       }

       variablesToRemove.add(variable);
   }

set操作里如果放进去的位置上原来已经被设置过值的话就会调用这个方法，但是我没看懂这个方法的作用是啥，还请老师指点。