Goal:
  
  A : Type
  H : ReflectEq A
  a1 : A
  l1 : list A
  a2 : A
  l2 : list A
  Heq : (a1 == a2) = false
  l3 : list A
  l : list A
  ============================
   (a2 :: l2 = l3
    → ([], a1 :: l1, a2 :: l2) = split_prefix (a1 :: l1) l3
      → let H := CoreTactics.block in
        is_prefix l (a1 :: l1)
        → is_prefix l l3
          → is_prefix l (fst (fst (split_prefix (a1 :: l1) l3))))


DEBUG:Going to execute:
cbv beta