+
+On platforms with the @code{UNWIND-TO-FRAME-AND-CALL-VOP} feature, it's
+possible to restart frames in the debugger, unwinding the binding stack.
+To know how much to unwind, @code{BIND-SENTINEL} in the beginning of a
+function puts the current frame pointer on the binding stack with
+@code{UNBOUND-MARKER-WIDETAG} instead of the symbol/tls-index.
+@code{UNBIND-SENTINEL} removes it before returning. The debugger then
+search for @code{UNBOUND-MARKER-WIDETAG} with the value being equal to
+the desired frame, and calls @code{UNBIND-TO-HERE}. Consequently,
+@code{UNBIND-TO-HERE} treats @code{UNBOUND-MARKER-WIDETAG} the same way
+as zeros.
+