protect against misleading results due to non-existent table

[Eric K]

When a referenced table does not exist, it is imputed to be empty.

For example if there was a rule:
safe_ip(x) :- ip(x), not dangerous(x)

But the table: dangerous does not exist, then safe_ip === ip, potentially misleading.

Do something about this. Some ideas:
- enforce table referential integrity
- indicate when results based on imputed-empty table
- indicate that computed table may be incomplete (some expected tuples may not be computed) and/or table may be unsound (some computed tuples may not be expected) with respect to what results "should be" if all referenced tables exist.