Change \retrograde to deal with ties and repeat chords/notes (issue 302470043 by address@hidden)

[thomasmorley65]

Didn't dive into the code in depth. From my (limited) testings, I'd say
it's very nice, thus:
LGTM

A nitpick:


https://codereview.appspot.com/302470043/diff/1/scm/modal-transforms.scm
File scm/modal-transforms.scm (right):

https://codereview.appspot.com/302470043/diff/1/scm/modal-transforms.scm#newcode188
scm/modal-transforms.scm:188: ;; See also LSR #105.
There is no LSR-snippet 105. Must have been deleted somewhen in the
past. Actually the entire LSR doesn't mention retrograde at all
nowadays.

https://codereview.appspot.com/302470043/