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:
cbv beta iota delta [TODO projection TODO projection] in id, id'
DEBUG LTAC Evaluated term: X1
DEBUG LTAC Evaluated term: (list A)
Level 3: type of id
 has value 
(list A)