create k
creates a handler by coupling the continuation k
together with the
evaluation context that is current at the time create
is called. Whenever this
handler is later invoked, it will happen in this saved evaluation context.
install h d
behaves like upon
except that it also returns a uninstall function
that, when called, uninstalls the handler
schedule h v
schedules the handler h
to run at some point in the future by being
called on value v