Goal:
  
  A : Type
  H : ReflectEq A
  x : A
  xs : list A
  y : A
  ys : list A
  tl : list A
  ============================
   (x = x ∧ is_prefix xs (xs ++ tl))


DEBUG:Going to execute:
<coq-core.plugins.ssreflect::ssrtclintros@0> arg