Emirp number

{
    EMIRP.PAS, computes numbers that are prime numbers when read backwards
    to forwards, too, e.g., 13 17 31 37 71 73 79 97. To compile run "fpc emirp.pas".
    Copyright (C) <February 01, 2016> Henning Polzer,
    send comments and error reports to: h underscore polzer at gmx dot de.

    This program is free software; you can redistribute it and/or
    modify it under the terms of the GNU General Public License
    as published by the Free Software Foundation; either version 2
    of the License, or (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program; if not, write to the Free Software
    Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
}

PROGRAM mirpzahlen_berechnen;
TYPE Tgz = int64;                   { Standard: integer }

VAR i, og, ug: Tgz;

PROCEDURE emirp (zahl: Tgz);
VAR h: Tgz;

  FUNCTION prim (zahl: Tgz): boolean;       { Primzahl? }
  VAR k: Tgz;

  BEGIN { prim }
    prim := true;
    k := 3;

    IF zahl / 2 = trunc (zahl / 2) THEN prim := false;
    IF zahl / 2 <> trunc (zahl / 2) THEN
      WHILE ((zahl / k <> trunc (zahl / k)) AND (k * k < zahl)) DO
        k := k + 2;
    IF zahl / k = trunc (zahl / k) THEN prim := false;

    CASE zahl OF
      1      : prim := false;
      2, 3, 5: prim := true
    END { case }
  END; { prim }

  FUNCTION spiegelzahl (zahl: Tgz): Tgz;
  VAR div_erg, s, z: Tgz;

  BEGIN { spiegelzahl }
    s := 0;
    z := zahl;

    IF z < 10 THEN s := z ELSE
      REPEAT
        div_erg := trunc (z / 10);
        s := (s + (z - div_erg * 10)) * 10;
        IF div_erg < 10 THEN s := s + div_erg;
        z := div_erg
      UNTIL z < 10;

    spiegelzahl := s
  END; { spiegelzahl }

BEGIN { emirp }
  h := spiegelzahl (zahl);
  IF h <> zahl THEN                 { (Primzahl)palindrom ausser acht lassen }
    IF (prim (zahl) AND prim (h)) THEN write (zahl, ' ')
END; { emirp }

BEGIN { Hauptprogramm }
  write ('Untergrenze: '); readln (ug);
  write ('Obergrenze : '); readln (og);
  i := ug;
  IF i < 13 THEN i := 13;

  REPEAT
    emirp (i);
    i := i + 1
  UNTIL i > og;
  writeln
END.