It can be derived, thanks to Ales for noticing!

when using iCombine.

TODO: document this.

Otherwise, the tactic will fail subsequently. Besides, it was inconsistent
w.r.t. the iLöb tactic, which was already doing this.

Now they can also be used to clear/frame the whole pure/persistent/spatial
context.

After discussing this with Ralf, again, it turned out that using a bar
instead of a turnstyle would be better. When formalizing type systems, one
often wants to use a turnstyle in other notations (the typing judgment),
so having the turnstyle in the proofmode notation is confusing.

This enables things like `iSpecialize ("H2" with "H1") in the below:

  "H1" : P
  ---------□
  "H2" : □ P -∗ Q
  ---------∗
  R