Natural_deduction : PRF_main.prf_raw_of_prf

let rec prf_raw_of_prf_seq ?(print_tokens = false) (prf_seq : PRF_sequencer.t_prf_seq) : t_prf_raw =
        match prf_seq with
        |Prf_seq s ->
                let lexbuf = Lexing.from_string s in
                try PRF_parser.main (lexer print_tokens) lexbuf with
                |_ ->
                        match print_tokens with
                        |false -> 
                                let _ = IO.print_to_stderr_red "PRF_parser failed; read the following tokens:" in
                                prf_raw_of_prf_seq ~print_tokens:true prf_seq
                        |true -> raise (Parse_error s)