Footnotes

 

(1)

This work was performed while the author was on sabbatical at the Wilhelm-Schickard-Institut für Informatik, Universität Tübingen.  

(2)

So far we haven't attached names to the project/problem suggestions other than the ones we posed ourselves, because of some uncertainty about who suggested them. If you want your name so attached, please let us know. It would be helpful also to send a paragraph describing the project or clarifying the problem.