-;;;
-;;; If the actual continuation is only used by the LET call, then we
-;;; intersect the type assertion on the dummy continuation with the
-;;; assertion for the actual continuation; in all other cases
-;;; assertions on the dummy continuation are lost.
-;;;
-;;; We also intersect the derived type of the CALL with the derived
-;;; type of all the dummy continuation's uses. This serves mainly to
-;;; propagate TRULY-THE through LETs.