[ Issue 1375 ] Fixed by more precise SIZELT analysis of clauseTel.

TODO: Make analysis even more precise.
[ Issue 1375 ] Variations on a test case.
[ testsuite ] Silenced succeed/Issue1345 (no debug output!).
[ issue 1293 ] Cosmetic changes
[ issue 1293 ] Show instance not used any more when reporting errors.

The instance is

  instance Show a => Show (Position' (Maybe a))

from Agda.Syntax.Position.
[ issue 1293 ] Added TODO
[ issue 1293 ] Replaced show function by pretty