comments on concepts doc

I attach a marked up copy of the concepts doc containing a lot of green 
ink.  The good news is that its green which makes it stylistic rather than 
substantive comment.  The bad news is that there is a lot of it.  I've only 
got about half way through.  I'll try to find time tomorrow to mark up the 
rest, but may have to steal it from working on schema with Danbri.

Brian