Crescendos and melismas break \partcombine?

[Karlin High]

Most music I print is for people used to SATB hymnals that have notes on 
common stems when possible. Unison parts have a single notehead with one 
stem up and one stem down. These conventions make me a heavy user of 
LilyPond's \partcombine and related features.

Now I am working on a piece with crescendos, one of which has a melisma 
for soprano but not alto. This seems to break the part combining, which 
results in unwanted extra note stems and crescendo marks. A code example 
is shown below. PNG output is attached.

I have tried using \partcombineChords instead of \partcombineAutomatic. 
"Chords" combines some things that "Automatic" is missing, but gets too 
aggressive with unison notes - they end up with 2 heads on 1 stem. And 
doing a \partcombineApartOnce seems to break the crescendo, with a 
warning about it being "unterminated."

Any idea how to keep the parts combined here? Thanks in advance
--
Karlin High
Missouri, USA

% BEGIN LILYPOND CODE
\version "2.18.2"
Soprano = \relative c' {
  % This line combines and crescendos properly
  e4 f g \cresc a b c \f d e
  % For this line, apparently due to the melisma eighth notes,
  % the parts de-combine. They get unwanted note stems and cresc. marks.
  e, f g \cresc a b8( d8) c4 \f d e
  % Without the crescendo, this line combines properly, even melismas.
  e, f g a b8( d8) c4 d e
}

Alto = \relative c' {
  c4 d e \cresc f g a \f b c
  c, d e \cresc f g a \f b c
  c, d e f g a b c
}

\score {
 \new Staff = "sopranoalto" << \partcombine \Soprano \Alto >>
}
% END LILYPOND CODE

cresctest.png
Description: PNG image