The return value of each command can also be a cons (:async . FETCHER)
where FETCHER is a function of one argument, CALLBACK. When the data
arrives, FETCHER must call CALLBACK and pass it the appropriate return
The return value of each command can also be a cons (:async . FETCHER)
where FETCHER is a function of one argument, CALLBACK. When the data
arrives, FETCHER must call CALLBACK and pass it the appropriate return