Bala

Copperbox revision 351.

I've reduced layering in type Event - Note Rest and Spacer now have their own construtors, previously they were grouped in the DEvt - durational event - type (Event itself is a new name previously it was called Elt). Mark is still a compound type within event, it is likely I will add more marks than Tie and also marks generally aren't interpreted - at least until they are passed to HNotate - so there is little need for nested pattern matching on them.