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    by passing in the same query parameters.

 C. Have the query_async() call take an extra parameter, a reference
    to a callback function... i.e. a function that will be called
    whenever the query finishes, with the result of the query.

I think I prefer strategy A to B; it's more standard of a practice.
One trick about A is knowing where to store the "token".
I'm not sure yet about A vs C.

C fits in more naturally with coroutines and continuations.
Might make the code more simple, but I need another function that
will block until the query is finished.