Goal:
  
  A : Type
  f : nat → A
  m : nat
  IHm : ∀ n : nat, n < m ↔ nth_error (unfold m f) n = Some (f n)
  Hn : m < S m
  ============================
   (nth_error (unfold m f ++ [f m]) m = Some (f m))


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