Compute solver name list in help message.

main.ml

@@ -119,9 +119,17 @@ let verify_cmd =
   let cmdname = "verify" in
   let docv_solver = "SOLVER" in
   let solver =
+    let solver_names =
+      List.map ~f:(fun s -> String.lowercase (Solver.show s)) Solver.defaults
+    in
     let doc =
-      "The solver to use for verification. \
-       $(docv) must be either `pyrat' or `marabou'."
+      Format.asprintf
+        "The solver to use for verification. \
+         $(docv) must be chosen among: %a."
+        (Format.pp_print_list
+           ~pp_sep:(fun fmt () -> Format.pp_print_string fmt ", ")
+           Format.pp_print_string)
+        solver_names
     in
     let solvers =
       Arg.enum