  In their paper "A Functional Abstraction of Typed Contexts", Danvy and
Filinski show how to derive a monomorphic type system of the shift and reset
operators from a CPS semantics. In this paper, we show how this method scales
to Felleisen's control and prompt operators. Compared to shift and reset,
control and prompt exhibit a more dynamic behavior, in that they can manipulate
a trail of contexts surrounding the invocation of previously captured
continuations. Our key observation is that, by adopting a functional
representation of trails in the CPS semantics, we can derive a type system that
encodes all and only constraints imposed by the CPS semantics.
