function substring
    (str : string; start : integer; finish : integer)
        return string
is
    package string renames ada.strings.unbounded;

    iter : integer := 1;
    buffer : string.unbounded_string;
begin
    for iter in 1 .. str'length loop
        if iter >= start and iter <= finish then
            string.append(buffer, str(iter));
        end if;
    end loop;

    return string.to_string(buffer);
end substring;