procedure ZenchiKihou(P : PTree);
          {前置記法で書く}
  begin
    if P <> Nil
      then begin
             Write(P^.Symbol);
             ZenchiKihou(P^.Left);
             ZenchiKihou(P^.Right);
           end;
  end; {ZenchiKihou}

または

procedure ZenchiKihou(P : PTree);
          {前置記法で書く}
  begin
    Write(P^.Symbol);
    if P^.Left <> Nil
      then ZenchiKihou(P^.Left);
    if P^.Right <> Nil
      then ZenchiKihou(P^.Right);
  end; {ZenchiKihou}