How to clear a vim register (vi/ex buffer)?

Question

In the ex linage of editors, one can append to a register(vim)/buffer(ex,vi) by specifying the register name in uppercase. As an example

g/pat/y A

will append every line match pat to register a. But if a already has content, it will need to be cleared first.

How?

In vim, one can do let @a = ''. But this depends on having a vim compiled with the expression evaluation feature +eval. One can also do qaq, to record nothing into a, but recording is disabled while executing a register, inside mappings or in :normal, so it does not lend itself to automating.

vi/ex has no let, and no q or 'recording' equivalent , so how is buffers(registers) cleared in practical use?

To my disappointment, :0y a put line 1 in a, so the only way I can gleam is doing "ay$ on an empty line. (EDIT: Apparently there is a discrepancy in how y$ works on empty lines between vim and vi)

Anything better?

Answer 1

So it turns out that all Vi clones are, well, different. For example, 0"ayh clears register "a" in Vim, but not in Elvis; while 0"ad0 works in Elvis, but not in Vim.

As Busybox seems to ship "Vi" of its own, you have no other choice than to experiment and to find the right keys yourself.