Goal:
  
  A : Type
  B : Type
  x : A
  a : A
  l : list A
  proj : A → B
  size : B → nat
  hin : In x l
  IHl : BinInt.Z.lt (BinInt.Z.of_nat (size (proj x)))
          (BinInt.Z.of_nat (list_size (λ x : A, size (proj x)) l))
  cstr : BinInt.Z.le BinNums.Z0 (BinInt.Z.of_nat (size (proj a)))
  cstr0 : BinInt.Z.le BinNums.Z0
            (BinInt.Z.of_nat (list_size (λ x : A, size (proj x)) l))
  cstr1 : BinInt.Z.le BinNums.Z0 (BinInt.Z.of_nat (size (proj x)))
  ============================
   (BinInt.Z.lt (BinInt.Z.of_nat (size (proj x)))
      (BinInt.Z.add
         (BinInt.Z.add (BinInt.Z.of_nat (size (proj a)))
            (BinInt.Z.of_nat (list_size (λ x : A, size (proj x)) l)))
         (BinNums.Zpos BinNums.xH)))


DEBUG:Going to execute:
idtac