filtering dynamic and note events separately

[Kieren MacMillan]

Hello, all --

Han-Wen recently talked about "filtering events" in some thread or  
other, which got my brain ticking...  =)

Is there already a function that would take a music variable and  
extract only the note or dynamic events? I'm brainstorming a way to  
(potentially) simplify piano-centered dynamics by having

    upperNotesPlusDyn = { blah, blah, blah }
    lowerNotes = { otherstuff otherstuff }

and then saying [pseudo-code]

    \new PianoStaff
    <<
        \new Staff = "upper" << #filter (note-event)  
\upperNotesPlusDyn >>
        \new Dynamics << #filter (dynamic-event) \upperNotesPlusDyn >>
        \new Staff = "lower" << #filter (note-event) \lowerNotes >>
    >>

This would save all the skipped notes usually necessary to get the  
Dynamics right. [Of course, in certain scores, that might be the  
easier way -- I'm just thinking about alternatives for simpler scores.]

Thanks,
Kieren.