code: private MutableDouble mutableRemove(E key) {
      MutableDouble md = map.remove(key);
      if (md != null) {
        totalCount -= md.doubleValue();
      }
      return md;
    }
docs: This is used internally to the class for getting back a
  MutableDouble in a remove operation.  Not for public use.
  
  @param key The key to remove
  @return Its value as a MutableDouble