#56 add multiplicative form to leadsheet notation

[Robert Keller]

Currently really long notes and rests take on a form something like this: r1+1+1+1+1+1 (representing 5 whole-note equivalents of rest). It would be helpful to have a more succinct notation such as r1*5 to make melody text more readable.

[Robert Keller]

Or 5*r1 might be better.