Goal:
  
  G : t
  A : Type
  B : Type
  f : A -> B -> A
  g : A -> B -> A
  hfg : f =2 g
  x : list B
  acc : A
  ============================
   (fold_left f x acc = fold_left g x acc)


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