Since we are not providing a generic command function that takes a write concern parameter (that is, there is no mongoc_client_command_with_write_concern), users must choose to manually append a "writeConcern" subdocument to the command document before calling a generic function like mongoc_client_command.

Let's help them:

bool 
mongoc_write_concern_append (mongoc_write_concern_t *write_concern,
                             bson_t                 *doc);

The passed-in write_concern is conceptually const, but not actually const so that we can call _mongoc_write_concern_freeze on it.