Is it possible to astrally attack foci in SR4?, and if so, what are the rules? Options

[Psikerlord]

Hi all, I seem to remember in SR2 that you could attack foci in the astral plane, break them and ground effects into the physical plane. I know you can't ground effects in SR4 (or SR3), but is it still possible to "attack" foci in the astral plane? I can't find any rules on it in the core book, or street magic, so it seems to me they can't - although they can of course be distrupted by passing thru wards. Foci are "astral constructs" according to street magic, but it doesn't elaborate further. The rules for making holes or passing thru wards don't seem very suited to "attacking" a focus. Perhaps it is a game balance matter, similar to the way in which a character's gun cannot normally be attacked, or his armour, etc..? (although I guess they can be, using called shots, perhaps?) Thanks in advance for any responses.

[Ancient History]

One of those things pretty much cut for space. Assume that the focus has barrier and structural ratings appropriate for its material and construction, and that it "heals" Force worth of damage every combat turn. The Disrupt (Focus) spell in Digital Grimoire also gives some guidelines.

[Psikerlord]

thanks for the pointers, AH