I composed a short Scottish-style reel on my Martin guitar using a three-string capo last week. I put the capo at the second fret covering the D,G and B strings, creating an open A tuning because the capo serves as the fingers that make an A chord. It's not the same as retuning the guitar because three strings are open and are difficult to play at the second fret and below with that capo in the way, but I learned fingering patterns that allowed me to play tunes in bagpipe style, and using that tuning I wrote the reel, which has yet to find a name for itself. I practiced that tune incessantly last night, for about 45 minutes, working to perfect the fingering. I can whip out my new tune on mandolin in seconds, but it's a challenge on a guitar with a partial capo, a challenge I enjoy. I know the tune so well that for me it has become part of the Scottish tradition, as firmly embedded in my mind as are tunes written long ago, and I'll probably include it on my recording.