
Chapter 6: On Domain Independent Databases 227
THEOREM 3 (TOPOR [1986])
Every allowed formula is safe and hence domain independent. •
The proof that every allowed formula is safe is, by structural induction,
using the similarity of the defining conditions. Second, allowed formulas can
be transformed into particularly simple equivalent relational algebra expres-
sions (Van Gelder and Topor [1987]). Finally, the evaluation of an allowed
query, with respect to a database using SLDNF-resolution as described in
Lloyd and Topor [1986], never terminates with a control error (i.e., it never
"flounders").
Other recursive subclasses of the domain independen ...