Choices for CR 20

There are basically three choices:

   0. Status quo.  Missing wsa:To in the infoset means [destination] ==
      anonymous in the MAPs, always.
   1. Limit this defaulting to the context of request-response.  If you
      want to use an anonymous [destination] elsewhere, you have to do
      so explicitly.
   2. Get rid of defaulting entirely.  You must always spell out what
      value you want for [destination].

Separately from this, we can place various restrictions on the use of
anonymous [destination], however it may have arisen, as part of
resolving CR 18.  For example, in /any /of the three cases, we could say
that anonymous [destination] is only allowed for response messages as a
result of section 3.4.  We could also ban anonymous [destination]
altogether.

Any such restriction is still compatible with the status quo.  The
status quo says what the default value of [destination] /is/, not when
it may or may not make sense to use this defaulting.  Put another way,
if I leave out wsa:To and it turns out I can't use anonymous, I'll get
an error, just the same as if I explicitly set wsa:To to anonymous.

In short, the resolution to CR 18 may affect the resolution to CR 20,
but not vice versa.

Unless we can /completely/ specify when anonymous [destination] is OK
and when it's not, it seems risky to try to scope the defaulting rule.