is_user (username : STRING; password : STRING) : BOOLEAN
    do
        from
            users.start
        until
            users.exhausted or found
        loop
            if(username.is_equal(users.item.username) and password.is_equal (users.item.password))
                then
                    result := true
            else
                users.forth
            end
        end
        result := false
    end