cleanup of the file incidence_structures.py
correct link to trac
Poset.is_chain is wrong and other details
Merge branch 'master' into ticket/15330
trac #15330: minor docstring edit
Poset.lt computes too much
Rebase on 5.13.beta2
fix documentation
fix latex
Trac #15562: PDF docs breakage in CachedMethod

URL: http://trac.sagemath.org/15562
Reported by: vbraun
Ticket author(s): Volker Braun
Reviewer(s): Simon King
Trac #15563: Illegal LaTeX in toric homset

URL: http://trac.sagemath.org/15563
Reported by: vbraun
Ticket author(s): Volker Braun
Reviewer(s): Simon King
Trac #15055: minor cleanup in incidence structures

this ticket aims to

* remove unused import statements in the file
combinat/designs/incidence_structures.py

* put this file in a pep8 standard shape

URL: http://trac.sagemath.org/15055
Reported by: chapoton
Ticket author(s): Frédéric Chapoton
Reviewer(s): Nathann Cohen
Trac #15330: Poset.is_chain is wrong

I will say nothing.

Nothing.

It may get messy otherwise.

Nathann

URL: http://trac.sagemath.org/15330
Reported by: ncohen
Ticket author(s): Nathann Cohen
Reviewer(s): Darij Grinberg
Trac #15332: Poset.lt computes too much

`Poset.lt(x,y)` also wonder whether `x>y` and nobody asked it to.

URL: http://trac.sagemath.org/15332
Reported by: ncohen
Ticket author(s): Nathann Cohen
Reviewer(s): Darij Grinberg