3004: watch for matching -arg in _combine_inverse
record expressions seen in match_commutative
3539a: match fails if pattern has extra syms

The "extra syms" are symbols in the exact part of a pattern. If the
symbols are combined with Wilds (as shown in the added test) then
the match continues.
3539: do not let Add match become more complex
Merge pull request #1685 from nthorne/doc-linkfix

Fixed dead links in documents
Add Niklas Thörne to the AUTHORS/aboutus/mailmap.  Welcome to SymPy!
add: a missing link
add chain branching strategy
add chain to imports
factor debug to onaction and write
onaction imports
add sfilter strategy
add triangular predicate (no logic yet)
add unit_triangular predicate
add implications for triangularity
add failing test for triangular implication
diagonal implies symmetric
add ask_update executable
add trailing comma to compute_known_facts
move ask_update to bin