Goal:
  
  P : elt -> elt -> Prop
  s : t
  H : forall v1 v2 : elt, In v1 s -> In v2 s -> P v1 v2
  x : elt
  H0 : In x s
  x0 : elt
  H1 : In x0 s
  ============================
   (P x x0)


DEBUG:Going to execute:
assert X2
Level 1: evaluation returns
assert X2; [ exact (id0 id1) | clear id0 ]
where
id1 := x
of type
intropattern
id0 := P
of type
intropattern
flags := 
of type
tauto_flags
X2 := elt -> Prop
of type
constr of type tacvalue