Issue 4419: Engraving ends too early (issue 252970043 by address@hidden)

[nine . fierce . ballads]

I haven't absorbed enough of the background knowledge on iteration to
say credibly that this looks good, although the C++ looks fine.

Because this is a second try at fixing a problem, perhaps a regression
test should be written or expanded to cover the case that the first try
did not.

https://codereview.appspot.com/252970043/