  Type systems usually characterize the shape of values but not their free
variables. However, there are many desirable safety properties one could
guarantee if one could track how references can escape. For example, one may
implement algebraic effect handlers using capabilities -- a value which permits
one to perform the effect -- safely if one can guarantee that the capability
itself does not escape the scope bound by the effect handler. To this end, we
study the $\textrm{CF}_{<:}$ calculus, a conservative and lightweight extension
of $\textrm{System F}_{<:}$, to track how values and their references can be
captured and escape. We show that existing terms in $\textrm{System F}_{<:}$
embed naturally in our calculus, and that many natural problems can be
expressed in a system that tracks variable references like we do in
$\textrm{CF}_{<:}$. We also give mechanized proofs of the soundness properties
of $\textrm{CF}_{<:}$ in Coq. The type system presented in $\textrm{CF}_{<:}$
is powerful enough to reason about safety in the context of many natural
extensions of $\textrm{CF}_{<:}$ such as region-based memory-management,
non-local returns, and effect handlers.
