Parry or Block for someone else

[kdtipa]

I couldn't find this in older threads, and I don't remember this in the book (though it's possible I missed it): Is there an official way to handle someone blocking or parrying an attack aimed at someone else?

If you imagine three combatants standing in three hexes so they can all reach each other, and for the example, they each have a sword and a shield. Person A and Person B are on the same side, fighting against Person C. Person C manages to hinder Person A in some way that makes them an easier target, so on their turn, they attack Person A and Person A is in trouble.

Person B is close enough to reach the other two, and in real life, I can imagine Person B trying to parry the attack or get their shield in the way to protect Person A. It happens in movies and shows often enough for me to think it's worth considering for GURPS too.

Are there rules for that? Is it the case that if they're in reach, they can just make a normal block or parry for the person they're protecting? Maybe at a penalty (-2 or something)? Obviously it would count toward their defenses that round. Maybe even give them a penalty to defending themselves since they had to reach to protect someone else?

Do the rules change if the attacker is trying to attack through the defender's hex to a hex behind the defender?

I guess, failing official rules, has anyone put together house rules that have worked well?