Goal:
    A : Type  H : ReflectEq A  x : A  l1 : list A  l2 : list A
  ============================
   (is_prefix l1 l2 → is_prefix (x :: l1) (x :: l2))


DEBUG:Going to execute:
<coq-core.plugins.ssreflect::ssrmove@1> $1 $2