Goal:
  
  A : Type
  size : A → nat
  a : A
  l : list A
  IHl : list_size (rev l) = list_size l
  ============================
   (list_size (rev (a :: l)) = S (size a + list_size l))


DEBUG:Going to execute:
<coq-core.plugins.ssreflect::ssrrewrite@0> $1 $2