Definition monotone2 T0 T1 (gf: rel2 T0 T1 -> rel2 T0 T1) :=
  forall x0 x1 r r' (IN: gf r x0 x1) (LE: r <2= r'), gf r' x0 x1.

Theorem paco2_mon: monotone2 (paco2 gf).