The problem is in bar 27. As you can see, I have already used "
\override Staff.NoteCollision #'merge-differently-dotted = ##t " and
now I need to use " \override Staff.NoteCollision
#'merge-differently-headed = ##t " for the b'' in the right hand. It
will be an eighth note in one voice and a half note in the other
voice. This piece is almost but not quite three part polyphony except
that the three voices are not continuous voices; one comes in as
another leaves, then a new voice comes in over top again. It seems to
be an exercise in merging voices! Perhaps this would be a good
illustration for a HOW TO sometime in the future.
You can set both NoteCollision overrides together; they're not mutually exclusive. Futhermore, unless you use \once, they're set until you change them back (either by = ##f or using \revert).
Regards,
Neil