Посреди четырехзначных чисел избрать те, у которых цифры

Program z1;
var x1,x2,x3,x4:byte;
begin
for x1:=1 to 6 do begin
        for x2:=x1+1 to 7 do begin
                for x3:=x2+1 to 8 do begin
                        for x4:=x3+1 to 9 do begin
                                writeln(x1,x2,x3,x4)
                        end
                end
        end
end;
readln
end.