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
[ trivial
with eq_true
| reflexivity
| symmetry; trivial
| contradiction
| congruence ]