How to use `global-measure.id`.

[rpatters1]

Now that id is a universal object property, there seems to be some tension between global-measure.id and global-measure.index. They both uniquely identify a measure global instance.

I have had some confusion over exactly how index should be used. Since index is neither 0-based nor required to be sequential, and since part-measure has no cross-referencing index property, I have been maintaining a mapping between index values and actual indices in the global measure array. Then I use the actual index to retrieve corresponding part measures (or vice versa). But now it seems like global-measure.id serves the same purpose.