pretty-printing of {within _, continuous _}

[affeldt-aist]

Co-author: holgerthies

fixes #1781

Motivation for this change

Checklist

• added corresponding entries in CHANGELOG_UNRELEASED.md

• added corresponding documentation in the headers

Reference: How to document

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[affeldt-aist]

@zstone1 we observed that we can preserve the correct pretty-printing of the notation {within _, continuous _} by protecting the type constraint with a definition

do you think this could be an appropriate solution to fix issue #758 ?

fyi: @CohenCyril

[CohenCyril]

The problem with this notation is that it is not compositional, there are many reasons why f could have domain a subspace topology, and not have been tagged with from_subspace. However it does preserve the notation `{within ...} if you typed it in the first place, so it is still an improvement. I will work on a more general solution based on displays at some point, meanwhile this is a strict improvement.

[affeldt-aist]

I understand that this is only a temporary and partial solution. I'll complete the PR asap.

[affeldt-aist]

@CohenCyril I have added a comment in the documentation to make it clear that the mechanism is a temporary, partial solution. Ok to merge?