outside-staff-priority with slur and hairpin

[Jonathan Wilkes]

Hello,
     I did a quick exercise typsetting a Schumann score, and I'm puzzled 
because to get the hairpin under the slur I had to turn up 
'outside-staff-priority for the slur to be greater than 250.  Otherwise, 
the snippet below doesn't work correctly.
     Reading the NR, it sounds like Lilypond just sorts in increasing 
order for outside-staff objects.  Why can't I just set 
'outside-staff-priority for the Hairpin to #1?

I also tried using 'avoid-slur (and turning off outside-staff-priority 
for the hairpin) but to no avail.

Thanks,
Jonathan

\version "2.13.3"

        \relative c' {
                \key f \major
                \once \override Slur #'outside-staff-priority = #251
                \once \override Slur #'outside-staff-padding = #0
                \once \override Slur #'height-limit = #2
                \partial 4 c( |
                f2~ f8 e[
                \override Hairpin #'rotation = #'(7 0 0)
                \override Hairpin #'X-offset = #-3
                \override Hairpin #'Y-extent = #'(1 . 0)
                \override Hairpin #'height = #0.4
                \override Hairpin #'outside-staff-priority = #1
                f a]^\< |
                c f)\! <f f,>2 e8 d |
                c
        }