For my understanding (ignoring the loop):

    between mark a and mark b: 'a,'b
    goto lines containing Abc: g/Abc
    recognizable substitute:   s/yy/\=i/
      = prefixes a variable (escaped with \)

A problem is that it will only replace the first `yy` on the line (if you add g, to make it s/yy/\=i/g, it replaces all of them, but with the same value of i.

How to make it increment multiple yy's on the one line?