mrk and id

[Yves Savourel <ysavourel@enlaso.com>]

Another question about <mrk>:

I wonder if the id attribute should not be required.

In real situations most tools will need to assign some kind of ID to it. It also causes issues when you have an <mrk> without id in the source and create the target: technically an id should link both, but that force the tool to change the source.

Thoughts?
-yves