t >>= f returns a computation that sequences the computations represented by two
monad elements. The resulting computation first does t to yield a value v, and
then runs the computation returned by f v.
ignore_m t is map t ~f:(fun _ -> ()). ignore_m used to be called ignore,
but we decided that was a bad name, because it shadowed the widely used
Pervasives.ignore. Some monads still do let ignore = ignore_m for historical