Part of the answer you are seeking is in: Is splitting keywords with HTML tags bad for ranking? (ignore the title) where I discuss/describe how Google handles keyword phrases. It will explain how Google sees the search world of terms, phrases, and so forth. It is short and well worth a read.
In short, you do not have to handle permutations as in your examples. However, you do need to concern yourself with whether, using your example, xyz and abc of (xyz) (abc) are terms that would be recognized or unique. If unique, then Google may not have enough information on the term but can make some guesses.
Since Google Scholar in 2008, Google understood the unique power of citations and language and not only rolled Scholar into it's algorithms and expanded it's understanding of language, it's usage, how language varies, terms, term usage variations, term to term relationships, term topics, even where new terms are coined, and so on, but how to tie content to search queries. Google understands target audiences too, reading level, and whether content is written by a scientist, an academic, and enthusiast, etc. It can even recognize the author if no author name is given but patterns can be recognized tying it to other signed content. Google can even take into account cultural differences, such as Football in the U.S. and Football in the U.K. It can easily recognize small cultural differences that would escape most of us.
What Google can do is rather astounding.
The notion of keywords is fading as a result but only partially. For example, if I have content on a particular topic, I should really use the appropriate terms to help Google properly asses the topic. But the need to echo all or several possible uses of a term has been gone for more than a decade and Google is far more advanced in recognizing the various term usage throughout the world. However, it does not hurt to use permutations where it is natural to do so. Just make sure that you are not loading your content to influence a machine. Google hates that.
What is most important is this:
- Create compelling content that people will want to link to.
- Create natural content designed for humans and not for machines.
- Use most important topic terms in title tag. (important)
- Make title tag conversational and compelling. (important)
- Use most important topic terms in h1 tag but do not duplicate title
- Make h1 tag conversational and compelling. (important)
- Use as many important topic terms in description meta-tag as makes
sense, but do not over do it. Google does not weigh these terms
within it's index, but will match them in searches. (important)
- Make description meta-tag conversational and compelling. (important)
- Use other header tags (h2, h3, ...) to identify sub-topics within the
main page topic and make them conversational. (important)
- Create back links where possible using most important topic terms and
some with sub-topic terms using conversational language. (important)
- Create internal navigational links and conversational cross links
within content, possibly within sidebars, and possibly at the bottom
of content for similar topics. (important)
- Ensure that links are complementary between topics except for
These are just off the top of my head, but should get you started okay. Do these, and your site should perform well in search. Of course, you will want to gather metrics and make necessary changes to improve performance. This is completely normal. No-one gets it right the first time out.
But I cannot emphasize enough that keyword loading has gone by way of the do do (an extinct bird). It is important to signal topic through proper and appropriate keyword usage, but leave the rest up to Google. Google is very good at knowing when a search for a particular term is best served by your content even if you have not used that term as it appears in the search query. It can be rather remarkable.