(* my function: *)
let rec sd_print_trace = function
[] -> Printf.printf "\n--end\n %!"; print_newline; ()
| ((s1,s2), _, _)::l -> Printf.printf " (%s,%s) => \n%!" s1 s2; sd_print_trace l;;

(*
Error: This expression has type (string * string * 'a * 'b) list
       but an expression was expected of type
         ((string * string) * 'c * 'd) list
       Type string * string * 'a * 'b is not compatible with type
         (string * string) * 'c * 'd
*)