ac_nfs st (step st m1 v1) (m2, r2, t)
| _, B.Bind (a1, (B.Abst (n1, w1) as b1), t1),
_, B.Bind (a2, (B.Abst (n2, w2) as b2), t2) ->
ac_nfs st (step st m1 v1) (m2, r2, t)
| _, B.Bind (a1, (B.Abst (n1, w1) as b1), t1),
_, B.Bind (a2, (B.Abst (n2, w2) as b2), t2) ->