sub sum {
    my ($term, $a, $next, $b) = @_;
    if ($a > $b) { return(0); }
    else { return( $term->($a) + &sum( $term, ($next->($a)), ($next->($b)) ) ); }
}

sub inc {
    my ($n) = @_;
    return( $n + 1 );
}

sub sum_cubes2 {
    my ($a, $b) = @_;
    return( &sum(\&cube, $a, \&inc, $b) );
}