WordPress, categories, post and navigation

Please help me as I can’t find any solution for my problem. You’re my last hope.
I have a photoblog in WordPress. Images are in posts which are assigned into categories (sometimes few categories).
My problem is navigation – previous and next for posts.
I’d like navigation will only work within one category.

And this can be achieved by:

<?php next_post_link('%link', 'Next post in category', TRUE); ?>

But problem is when post is assigned to two categories because WordPress automatically assigns post to the category with lower ID. So I don’t have any influence on navigation since then because I can’t choose the category.
There’s some hope – I found plugin which is called Smarter Navigation http://wordpress.org/extend/plugins/smarter-navigation/

It has function


which can retrieve the category assigned to the post.
But the problem is that I don’t know PHP and have no idea how to use it.

On WordPress forum there is one example how this can be used:

function is_category_or_referrer($name) {
    return is_category($name) || $name == get_referrer_category()->name;

<?php if ( is_category_or_referrer('Paintings') ) {
    echo " class='current_page_item'";
} ?>

But I don’t know how to make it work with:

<?php next_post_link('%link', 'Next post in category', TRUE); ?>

so the navigation will work only inside chosen by me category (not the one with lower ID).

Anyway is this possible?
Or maybe you can find another solution for this problem?

I’ve found the solution - I had to disable multicategories :confused:
Now it’s ok.