"C x A" is a combination of C and A. C, x, and A are defined once somewhere at the beginning of the paper/book.
Should I take this (which doesn't actually hold in the general case) to mean that every document uses its own language? That isn't exactly good for readability. We certainly don't consider every software project to be written in its own language (and no, the semantics of C does not tell us what the dP function does).
You don't expect to understand Java after learning PHP, and you don't expect to understand a Topology paper after learning Algebra.
"C x A" is a combination of C and A. C, x, and A are defined once somewhere at the beginning of the paper/book.