Global Opaque P_e.
Definition P'_e := from_11_to_20 P_e.
Definition Y (z w y:bs) : bs :=
P (P z w) y.
Definition Y_e : BC :=
comp 2 1 P_e
[comp 2 0 P'_e [proj 2 0 0; proj 2 0 1] nil]
[proj 2 1 2].

Lemma Y_skipn z w y :
Y z w y = skipn (length w - length z) y.