dill: printf when rendering to non-default session
As per the note a couple lines up, +fore depends on drum semantics being
active. We can only guarantee those being present for the default
session, not for any others. So, we print a warning when appropriate.
:: perhaps +send this to .ram instead?
::
|= liz=(list (list @c))
+ ~? !=(%$ ses) [%d %foreing-in-session ses]
^+ +>
=. +>
=| biz=(list blit)
