opaque type proposal

[Alan Bawden <ALAN@MC.LCS.MIT.EDU>]

I'm still worried about letting inheritance slip into the opaque type
proposal.  It's not that I see any problem with Pavel's extension (I
don't), it's that I am worried that by deciding now on a semantics for
single inheritance we reduce our options if it ever becomes clear how
multiple inheritance should work.  I confess I don't have a good example of
how it might get us in trouble, but the transition from single to multiple
inheritance is such a hot topic these days that I don't think any of us can
really be certain where it might take us.