use List::Util qw( reduce );

sub seq_prod {
    no warnings 'once';
    reduce { seq_prod2($a,$b) } @_ ;
}

sub seq_prod2 {
    my ($s, $t) = @_;
    my @sval;
    sub {
        my @tval;
        while ( !@sval || !(@tval = $t->()) ) {
                return () unless @sval = $s->();
            }
        ( @sval, @tval );
    };
}

sub iter_prod {
    seq_to_iter( seq_prod( map iter_to_seq($_), @_ ) );
}