Goal:
  
  G : t
  HI : invariants
  HG : acyclic_no_loop
  x : V.t
  n : Z
  y : V.t
  He : EdgeSet.In (x, n, y) (E G)
  xl : Z
  eq : lsp (s G) x = Some xl
  pos : 0 <= xl
  yl : Z
  eq' : lsp (s G) y = Some yl
  ylpos : 0 <= yl
  ============================
   (xl + n <= yl -> xl + n <= yl)


DEBUG:Going to execute:
let rec do_ccl :=
            try do_atom; repeat (do_intro; try do_atom); try arith_congr;
             (solve [ split; do_ccl ])
           in
           (solve [ do_atom | use_hyps; do_ccl ]) ||
             fail "Cannot solve this goal"