DTD for Instruments.xml

This document is currently for information only, and has not yet been
implemented into Instruments.xml. It does, however, define the file
structure, and will be used as the basis for an XSD once any wrinkles
have been ironed out. Please make me aware of any errors or
inconsistencies you may find. The DTD has been validated by an online
validation checker, but I am making no guarantees as to it's accuracy at
this stage - Church Organist