Add test for issue 2811: (1/(1-t)).subs(t,1) returns zoo after a078ac2
give Dummy a sort key
Merge pull request #3002 from smichr/dsort

give Dummy a sort key
Merge pull request #2967 from skirpichev/2811-test

Add test for issue 2811: (1/(1-t)).subs(t,1) returns zoo after a078ac2
rework handling of re/im in solve
raise error if Abs-arg is not real or imag
Merge pull request #2927 from smichr/abs

issue 4046: better handling of Abs by solve
check that args to sort are Number-free