If I use Guru Davinder to avoid the net damage after a chum+Neural Katana, do I have to pay the 4 credits twice, once for Neural Katana and once for chum, or can I avoid all 6 net damage at once and pay just four to not trash the Guru?

Consult the Book of Armaments FAQ Timing Chart!

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3. The Runner encounters a piece of ice. (‘When encountered’ conditionals meet their trigger conditions)
3.1 Icebreakers can interact with the encountered ice,
3.2 Resolve all subroutines not broken on the encountered ice.
...Either the run ends: You are no longer encountering this ice, go to [6]
...Or the run continues: The encounter ends. The ice is PASSED. If there is another piece of ic e protecting
the server, go to [2]; if there is not another piece of ice protecting the server, go to [4].

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Guru Davinder:

Prevent all net and meat damage.

Whenever Guru Davinder prevents at least 1 net or meat damage, trash him unless you pay 4.

Chum:

The next piece of ice the Runner encounters during this run has +2 strength. Do 3 net damage unless the Runner breaks all subroutines on that piece of ice.

Neural Katana:

Do 3 net damage.

So the first thing to notice is that it's not Chum+Neural Katana. It's Chum and Neural Katana.

There are two separate effects. The first is from Chum, predicated on whether or not you break all subroutines on the ice. Since that (assuming you didn't) would take place at 3.1, Chum would inflict three net damage. Guru Davinder would prevent it since he is a Constant Ability without any language that would make it optional, and you can either pay the $4 or he stops returning your vidcalls. Then, in 3.2, the unbroken subroutine of Neural Katana fires off. Three more net damage, another $4 to the Temple of the Liberated Cred Account.

Yeah, except that the way Chum works, you can't follow the timing chart, since you can't resolve Chum's subs until after you encounter the next ice and break it's subroutines, or not... As soon as I don't break all the subs on the next ice, both Chum's subs & the second ice's subs fire (simultaneously?).

Let's say an ice had two subs that were both "do 3 net damage". Would you have to pay Guru Davinder twice if you didn't break them? If not, then why should you pay twice when Chum's subs and the next ice's subs fire? They fire at the same time! The timing chart doesn't work with Chum because you can't resolve it's subs until after you've encountered the next ice. It doesn't explicitly say in the timing chart at the end of 3.1 anything like "all if-all-subroutines-aren't-broken conditions are met..."

J

When you encounter Chum and do not break it, you resolve its subroutine then and there. It fires. The effect of the subroutine is not to deal damage; the subroutine instead creates a Conditional Triggered Effect that is locked and loaded. Its trigger is contingent on what happens in the next encounter.

And what happens next is dealt with explicitly in the official FAQ:

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75 Chum

• If the Runner jacks out immediately after encountering a Chum and not breaking its subroutine, the Runner does not take any damage.
• Chum deals its net damage the first time either of the following occur during an encounter with the next piece of ice:

1. step 3.1 of a run closes and there is an unbroken subroutine on the piece of ice.
2. step 3.1 of a run was skipped or did not resolve because the encounter/run ends (if 3.1 does not happen, no subroutines can be broken). This applies even if the piece of ice has 0 subroutines.

Regardless, two separate card effects = two sources of damage. And yes, if you hit a single piece of ice with multiple damage subroutines, you absolutely do treat them as separate sources of damage. The unbroken subroutines fire one at a time, and Guru Davinder affects them all separately. The only time you treat multiple damage sources collectively is if one of them uses the word "additional," then it is treated as an increase to the original damage source.

In case it's not evident, Guru Davinder sucks.

Grimwalker, Dude, you are soo correct. Thank you for making this clear to me. I agree with your precise analysis 100%...except that after having gotten 'boomed' with a breaking news-accl.diag.-combo on the second turn (!) in the first round at the last tournament, I don't think the Guru sucks at all! Four credits to avoid all meat damage completely disables many (lame) killer-combos like that.

Cheers!

btw, so if I understand this correctly, than for an ice with two net damage subs, which get resolved separately, you'd have to also pay the guru twice, correct?

cheers again.

J

Yep. Two subs, two damage sources.

GD does put a spanner in the works for many combos, but make sure you have enough money to avoid a Hatchet Job or Enforcing Loyalty trace! A number of BOOM combo decks pack such tools to get around runner defenses like GD, Plascrete, and NACH.

But, anything that makes the corp's combo incrementally harder to pull off is potentially worthwhile. Where GD suffers is against Jinteki, which will ping you with small sources of damage until you're pretty well tenderized. Imagine faceplanting a Komainu with GD on the board and a full hand...