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:
<coq-core.plugins.ltac::contradiction@0> $1
Level 0: Backtrace:

No primitive equality found.