Goal:
  
  A : Type
  H : ReflectEq A
  y : A
  ys : list A
  _x_ : A
  _xs_ : list A
  tl : list A
  ============================
   (∃ l' : list A, ys ++ tl = ys ++ l')


DEBUG:Going to execute:
match goal with
           | H:¬ _ |- _ => solve [ case H; trivial ]
           end
Level 1: Backtrace:

No such assumption.
Level 0: Backtrace:

No such assumption.
Level 0: Backtrace:

No such assumption.
Level 0: Backtrace:

No such assumption.