passed to the error-display-handler during expansion
(not just the exn records that actually get raised)
the motivation is to do a better job with TR's way of signalling
mutiple error messages.
Specifically, allow a name to be specified with an
extension such that subsequent uses of those extensions
will replace mixins instead of adding new ones
being edited in DrRacket (via places)
Added an API to let tools have access to that
information (and compute more stuff)
Used that to make an online version of Check Syntax
which led to a separately callable Check Syntax API.
