Basic Scheme Markdown Function

[Steph Phillips]

Hi all, this one's probably a simple solution, but I'm a little fuzzy on 
how to implement custom Scheme functions still.

I'm working on a musical theater score, and there are lots of instrument 
changes formatted like this:

changeBbClar = ^\markup {
    \general-align #X #-0.7
    \bold
    \override #'(box-padding . 0.5) \box
    "Bb Clarinet"
}

My end goal is a function where I can define the name of the instrument 
and the x-alignment - something like

\InstrumentChange "Bb Clarinet" #-0.7

Thanks a ton,
~Steph