sub sis {
    my ($self) = @_;
    
    # check whether the internal representation is already defined
    if ( !$self->{_sis} ) {
        # call the constructor and save the result
        my ($dbh, $uname) = get_constructor_args_from_somewhere();
        $self->{_sis} = Company::Dept->new($dbh, $name);
    }

    return $self->{_sis};
}