Goal:
  
  A : Type
  x : A
  y : A
  a : A
  l2 : list A
  H : [x] = a :: l2 ++ [y]
  H1 : x = a
  H2 : [] = l2 ++ [y]
  ============================
   ([] = a :: l2 ∧ a = y)


DEBUG:Going to execute:
subst