code: <T> Deque<T> getValues(Class<T> key) {
          Deque<T> values = (Deque<T>) contextValuesPerKey.get(key);
          if (values == null) {
              values = new LinkedList<>();
              contextValuesPerKey.put(key, values);
          }
          return values;
      }
docs: Determine the stack for the given key.
  
  @param key
  The key.
  @param <T>
  The key key.
  @return The stack.