program Find_All_Lower_Case_Characters;

const String_Size = 30;
type Low_Set = set of 'a'..'z';
var Data_Set    : Low_Set;

    Storage     : string[String_Size];

    Index       : 1..String_Size;

    Print_Group : string[26];
begin  (* main program *)

   Data_Set := [];

   Print_Group := '';

   Storage := 'This is a set test.';
   for Index := 1 to Length(Storage) do begin

      if Storage[Index] in ['a'..'z'] then begin

         if Storage[Index] in Data_Set then

            Writeln(Index:4,'   ',Storage[Index],

                         ' is already in the set')

         else begin

            Data_Set := Data_Set + [Storage[Index]];

            Print_Group := Print_Group + Storage[Index];

            Writeln(Index:4,'   ',Storage[Index],

                         ' added to group, complete group = ',

                         Print_Group);

         end;

      end

      else

         Writeln(Index:4,'   ',Storage[Index],

                       ' is not a lower case letter');

   end;

end.  (* of main program *)


Result of execution

   1   T is not a lower case letter

   2   h added to group, complete group = h

   3   i added to group, complete group = hi

   4   s added to group, complete group = his

   5     is not a lower case letter

   6   i is already in the set

   7   s is already in the set

   8     is not a lower case letter

   9   a added to group, complete group = hisa

  10     is not a lower case letter

  11   s is already in the set

  12   e added to group, complete group = hisae

  13   t added to group, complete group = hiseat

  14     is not a lower case letter

  15   t is already in the set

  16   e is already in the set

  17   s is already in the set

  18   t is already in the set

  19   . is not a lower case letter