Goal:
  
  A : Type
  H : ReflectEq A
  a1 : A
  l1 : list A
  a2 : A
  l2 : list A
  Heq : (a1 == a2) = false
  l : list A
  Heqcall : ([], a1 :: l1, a2 :: l2) = split_prefix (a1 :: l1) (a2 :: l2)
  ============================
   (is_prefix l (a1 :: l1)
    → is_prefix l (a2 :: l2)
      → is_prefix l (fst (fst (split_prefix (a1 :: l1) (a2 :: l2)))))


DEBUG:Going to execute:
<coq-core.plugins.ssreflect::ssrtclseq@0> tac dir arg
DEBUG LTAC Evaluated term: nil_prefix