using \include for fret diagrams

[Shelagh Manton]

I've been thinking about using fret diagrams in various lead sheets for
singing and finding it awkward to use fret diagrams. If I wrote a fret.ly
file with my usual set of frets in it can I include it as you would files
such as english.ly etc? 

Actually I know you can do this; probably what I'm asking is what would
the file look like? 

Shelagh