[ruina]

mas compacto aun:

Código Delphi [-]
procedure TForm1.Button2Click(Sender: TObject);
const a ='abcdef';
var s:string;
    procedure rec (i,j:integer);
    begin
          s[i]:=a[j];
          memo1.lines.add(s);
          if i<=length(s) then
             if j=length(a) then rec (i+1,2)
             else rec (i,j+1);
    end;
begin
  s:='aaa';
  rec(1,1);
end;