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Note sure whether these premises are the weakest possible.

Also, persistent stuff goes before plain stuff.

We do not have a notation for `bi_affinely` either, so this is at
least consistent.

(□ P) now means (bi_bare (bi_persistently P)).

This is motivated by the fact that these two modalities are rarely
used separately.

In the case of an affine BI, we keep the □ notation. This means that a
bi_bare is inserted each time we use □. Hence, a few adaptations need
to be done in the proof mode class instances.

(All the later lemmas are now prefixed by later_, and dito for
laterN, and except_0).

The absence of this axiom has two consequences:

- We no longer have `■ (P ∗ Q) ⊢ ■ P ∗ ■ Q` and `□ (P ∗ Q) ⊢ □ P ∗ □ Q`,
  and as a result, separating conjunctions in the unrestricted/persistent
  context cannot be eliminated.
- When having `(P -∗ ⬕ Q) ∗ P`, we do not get `⬕ Q ∗ P`. In the proof
  mode this means when having:

    H1 : P -∗ ⬕ Q
    H2 : P

  We cannot say `iDestruct ("H1" with "H2") as "#H1"` and keep `H2`.

However, there is now a type class `PositiveBI PROP`, and when there is an
instance of this type class, one gets the above reasoning principle back.

TODO: Can we describe positivity of individual propositions instead of the
whole BI? That way, we would get the above reasoning principles even when
the BI is not positive, but the propositions involved are.

Otherwise, ownership of cores in our ordered RA model will not be persistent.

As Aleš observed, in the ordered RA model it is not, unless the order on the
unit is timeless.

Thanks to discussions with Ales and Amin.