| By: |
(BackThru Thm* l1,l2:T List. l1 l2  ||l1|| ||l2||) THEN (Unfold `w-rcvs` 0)
THEN
(BackThru Thm* P:(T  ), L2,L1:T List. L1 L2  filter(P;L1) filter(P;L2))
THEN
(BackThru Thm* f:(A B), L1,L2:A List. L1 L2  map(f;L1) map(f;L2))
THEN
(BackThru Thm* i,j: . i j  upto(i) upto(j)) |