tutorial/latex: Remove redundant explanation
first commit for ticket 8723 : multiplication in elliptic curves
trac #8723 : cleaned the doc
Remove is_unit() from sage.structure.element.Ring and move it to the category of Rings

Before, the default implementation of is_unit() was not picked up by some
fields, e.g., some residue fields. Now this works correctly.
Prepare hash and copy for immutable graphs. Let .weighted() respect
mutability.
Use ecl_set_option instead of patching to disable ECL_OPT_TRAP_SIGCHLD
test that no SIGCHLD handler was installed
clarify test that no SIGCHLD handler is installed
very minor change, just to ping the branch
Merge branch 'trac8723' into new8723
Merge branch 'master' into ticket/15278

* master: (162 commits)
  [FIXUP] 5.13.beta4: update git specific files
  5.13.beta4
  5.13.beta4
  [FIXUP] 5.13.beta3: update git specific files
  5.13.beta3
  5.13.beta3
  Trac #15402: fix eint1() precision
  #14706: fix visibility of symbols for built-in libintl.
  also remove untracked directories after an aborted import patch
  trac_interface: encode fields with utf-8 as well
  sage.dev: don't textwrap during doctesting
  patch.py: deal with hg's silly copy commands
  remove reference to sage/misc/hg from docbuild
  SageDev: fix pull doctest
  SageDev: remove branch keyword from pull
  fix doctest errors in SageDev.push
  remove (now) bad uses of SageDev.pull
  remove unused garbage
  dumb down pull to fix it
  don't abort ticket editing on nop push
  ...
Merge branch 'u/ncohen/15491' of git://trac.sagemath.org/sage into ticket/15278

# By Nathann Cohen
# Via Nathann Cohen
* 'u/ncohen/15491' of git://trac.sagemath.org/sage:
  trac #15491: addressing the reviewer's comments
  trac #15491: directed immutable graphs report twice too many edges
Make digraphs using the static backend immutable and hashable
allow constant f in hyperelliptic curve constructor
reviewer's patch, makes sure that the parents are correct even with caching
streamline construction of PARI gen
raise an error instead of creating a NULL object in gen.__init__()
trac #15551: Rename Graph.trace_faces to Graph.faces
* First do checks for non-squareness that do not require factoring.
* Use PARI otherwise.
enable single-argument gcd/lcm