It turns out we *do* support the use of anonymous commands
(while it is not clear yet if we really need this feature).
Basically, client code may either create and register a new
instance from another command used as prototype, by invoking
Command::storeDef(ID). Or, alternatively it may just invoke
newInstance() on the command, which creates a new handle
and a valid new implementation (managed by the handle as
smart-ptr), but never stores this implementation into the
CommandRegistry. In that case, client code may use such a
command just fine, as long as it cares to hold onto that
handle; but it is not possible to retrieve this command
instance later by symbolic ID.
In the light of this (possible) usage pattern, it doesn't
make sense to throw when accessing a command-ID. Rather, we
now return a placeholder-Symbol ("_anonymous_")
Seems this was part of the confusion when looking at
the inheritance graph: Names where almost reversed
to the meaning. the ArgumentHolder was *not* the
argument holder, but the top level closure. And
the class "Closure" was not "the" Closure, but
just the argument holder. ;-)
