Goal:
  
  A : Type
  a : A
  l : list A
  l' : list A
  IHl : chop #|l| (l ++ l') = (l, l')
  ============================
   ((a :: l, l') = (a :: l, l'))


DEBUG:Going to execute:
(solve [ do_atom | use_hyps; do_ccl ]) ||
             fail "Cannot solve this goal"