Goal:
    A : Type  P : A → Type  a : A  X1 : list A  X : All P X1
  ============================
   ([a] = X1
    → let H := CoreTactics.block in let H0 := CoreTactics.block in P a)


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 := [a]
of type
constr
id'1 := X1
of type
constr
id' := X
of type
constr
id := X
of type
constr
cont := fun id id' => DepElim.decompose_exists id id'
        where
        id := X
        of type
        constr
of type
tacvalue of type tacvalue