Vim syntax patch

How to make a syntax patch for vim, by which I mean you can build some sugar on top of an already syntax-highlighted file format. I patched this onto YAML, and the trick is to stick it in ~/.vim/after/syntax/yaml.vim so it gets loaded last.

It looks like this: https://imgur.com/g3xX6lr

if matchstr(bufname("%"), "25-fw-names") != ""
  syn region A start=/- [adr]=[0-9]/ end=/^/ contains=C,E,D
  syn match C /[a-z]\+\ze=/ contained
  syn match D /=\zs[^=]\+\ze / contained
  syn match E /=\zs[^=]\+\ze$/ contained

  hi def link C Type
  hi def link D Keyword
  hi def link E Constant
endif