Goal:
    A : Type  P : list A → A → Type  x : A  a1 : list A  a : All_fold P a1
  ============================
   ([x] = a1
    → let H := CoreTactics.block in let H0 := CoreTactics.block in P [] x)


DEBUG:Going to execute:
try rename id into id',id' into id
Level 3: evaluation returns
try rename id into id',id' into id; cont id'1 id
where
t := [x]
of type
constr
id'1 := a1
of type
constr
id' := a
of type
ident
id := a
of type
ident
cont := fun id id' => DepElim.decompose_exists id id'
        where
        id := a
        of type
        ident
of type
tacvalue of type tacvalue