Trac #9321: add warnings to sum() and symbolic_sum() documentation
examples when/not to use Sage sum(); same also in calculus.py
fix doctests
moved warning after some examples
Ignore files from XCode
Added files from new XCode
Fixed a warning
Ensure that Sage is read/write
This patch fixes two things, both related to exclusion of invalid plot points.

 1. automatically exclude invalid points from a plot
 2. fix excluded regions in case more than one exclude point was in an invalid region
Thanks to kcrisman and ddrake for the idea how to do this auto exclusion
fix parametric plots that fail due to invalid plot points
Merge branch 'develop' into t/13246/automatic_exclusion_of_non_domain_points_in_things_like_arcsec
Merge branch 't/16439/allow_inverse_trig_functions_to_all_take_complex_input' into t/13246/automatic_exclusion_of_non_domain_points_in_things_like_arcsec
add more doctests for auto exclusion functionality
Expose bug #16485 in doctest.

At the moment this bug causes an exception from
sage/rings/polynomial/multi_polynomial_libsingular.pyx:
TypeError: keys do not match self's parent
Fix type mismatch in variety computation.

Switch all polynomials of triangular decomposition to target ring before
enumerating the variety.  This avoids mismatchs between rings.
Drive-by improvement: Omit multiplicities.

Instead of ignoring the multiplicities during iteration, it might be cleaner
to not ask for them in the first place.
fix latex according to reviewer comments
do not use exclude both as list and as None
sort and reverse the excluded_points in one command
fix parametric and polar plots if exclude point is outside the domain