collision

There is no such thing as a regulatory rule to give right of way to gliders climbing below. The common and general rule is to "see and avoid".

US Federal Aviation Administration (FAA) in Regulation 14 CFR Part 91.113: "When weather conditions permit, regardless of whether an operation is conducted under instrument flight rules or visual flight rules, vigilance shall be maintained by each person operating an aircraft so as to see and avoid other aircraft. When a rule of this section gives another aircraft the right-of-way, the pilot shall give way to that aircraft and may not pass over, under, or ahead of it unless well clear."

EASA/SERA Section 3 Chapter 2: "Regardless of the type of flight or the class of airspace in which the aircraft is operating, it is important that vigilance for the purpose of detecting potential collisions be exercised on board an aircraft. An aircraft shall not be operated in such proximity to other aircraft as to create a collision hazard."

If one can see another one is too close and considers collision is possible, no matter the rights of way, he shall avoid the collision.

I we want the one below to be the responsible for a collision, fine. But it is not due the a possible right of way of the one above. It is due to the fact he could see and had to avoid the collision.

Common sense.

We are not the first airsport discipline to have the problem. Gliding had an accident rate at high level competitions that were 10x more than regular solo flying (see link below for the analysis). Would be interesting to do this level of analysis on the problems paragliding/Hang Gliding face https://www.fai.org/sites/default/files/documents/collision_risk.pdf

I fully agree with Maxime Bellemin

In the end of the day, we want to be alive, not to be the person who is "right" in a mid-air.

For the mid-air NOT to happen you need "two to dance". Spatial awareness and curtesy is the solution, fixed rules do not work as well.

Especially in paragliding, fixed rules can be counterproductive .

Some times you can be the guy "below" and be in control, because you have seen the guy above you, you keep him / her in sight, even if momentarily they are invisible right over your canopy, you still know where they are, and you are "in control"

Some times, it is exactly the opposite. You are "in control" because it happened that you are above, the guy / girl that came below you on their "dead spot" and you know that.

Curtesy and Chivalry. Now you are responsible both for 'you' and for 'them', till they see you, so they can assume their part of responsibility.

It cannot be described with strict rules, it cannot be straightjacketed in Python scripts and algorithms.

Is "Paragliding".

It is fluid, it is chaotic, it flatty denies simple solutions. (Like "80 pilots")

Sorry to say, that means nothing. I'd rather be with 120 proficient pilots in the same thermal than 80 aggressive uncourteous chaps in 2 square kilometers spread...