Drop obsoleted comment about with statement in risch.py (closes issue 3665)
Preprocess user input for rsolve to allow linear recurrences in a more generic form (e.g. y(n) + a*(y(n + 1) + y(n - 1))
Fix issue 3565

    1) for |a| or |b| = oo: handle the case of geometic series
       with eval_sum_hyper()
    2) in doit(): work arround the case when eval_sum returns
       partially evaluated sum with Piecewise
Remove ancient, commented out, code from Sum.doit()

    This fragment is in place long time
    ago (commit 157dd374e728e37eebef729584cd9e94fad02bc8,
    issue 577) for no reason.
Merge pull request #1904 from skirpichev/misc

issues 3565 and 3665; preprocess rsolve input