My former post-doc host failed to credit my work in a paper - what to do? I was employed as a post-doctoral fellow at some institute, and left it a couple of years ago. When I left, it was on ok terms with my host (= person who invited me and arranged for funding for my stay) - although our relationship had not always been on the positive side, and we ended up collaborating only here-and-there rather than closely. We both have a negative view of some aspects of the other's personality.
Anyway, I've noticed that members of the research group - some junior ones, but also my host - have published a certain paper a few months. This paper is mostly not about my own work, but it presents a certain concept which (*) I was studying and developing; these ideas are now in work that I've published - but mostly on ArXiV and not in peer-reviewed venues. I've also sent an email link and an invitation to read it to a bunch of people in my field - including my post-doc host and one of his junior co-authors.
The paper does not mention any of the above; makes no reference to my work or to me; and says something like &quot;In this paper we introduce the concept of X&quot;.
Now, my work is much wider than just that concept (it fits into a larger theoretical framework); and the paper does something with this concept which I didn't explore nor write about. So it's not a case of brazenly stealing results.
A final point is that this work of mine has not yet gotten any (non-self) citations, which makes this situation sting more.
My questions:

Considering the circumstances, should I even get in contact with my former host and/or other co-authors about this, or should I just drop it? Especially seeing how this paper is already published and has made its rounds for a few months? I am worried that this will just develop into a fight involving third parties, a sort of a &quot;crusade run remotely&quot; on my part, and end up hurting more than it helps.
Assuming I do contact my former host about this - How diminutive/coy/reserved should I be in a first email about this?
What can I / should I ask for as a rectification of the situation?

(If you'd like more information, please ask in the comments.)
First, please remember that human memory is fallible. People have honestly forgotten work done by their students or their postdocs or even themselves. So I'd be inclined to assume, unless there's evidence to the contrary, that your supervisor has forgotten your work on the broader concept Y that subsumes the &quot;new&quot; concept X, and that he hasn't had time to study the arXiv preprint that you sent him.
Second, what should you do about it? I think that, under the assumption that no malice was intended, you could write your former supervisor an email along the lines of &quot;It seems to me that the concept X in your paper &quot;[title]&quot; is an instance of the general concept Y studied in my paper &quot;[title]&quot; [arXiv link] which I sent you a preprint of a while ago. I hope that my work on Y will be useful to you if you contiue to explore X.&quot; It seems to me that something like this should alert him to pay attention to Y and cite you in further work on X.  (Of course, you can put personal things like &quot;I hope you are well&quot; before and/or after the main message.)
I think you're unlikely to get a citation in the already published paper (unless it has an on-line version that gets updated), but if your supervisor produces more papers about X, you might get some citations in them.