Replace s::set ... from Kirby 2 with Kirby 4 stuff

[Carsten]

What was that s::get() and s::set(...) in Kirby2 , and what can I replace it with in Kirby4?

[texnixe]

Kirby 2 docs are still available in the archive: https://v25.getkirby.com/

@Carsten

s::get() and s::set(...)

Setter and getter for session values.

$session