Discussion of substitution groups

Hello UBL TC,

Remember that we are discussing substitution groups (and possible
alternatives to the same) as a code list extension mechanism in
next week's Atlantic TC call.  Given that Mark Crawford will be
unable to attend, it would be good to have as much information as
possible (including the outcome of discussions with Henry
Thompson) posted to this list prior to that meeting, preferably at
the beginning of the week.

Jon