The interface treats NULL's by introducing a single valued function
'NULL'/1 whose single value is a unique 
Skolem
constant.
For example a NULL value may be represented by
'NULL'(null123245)Under this representation, two distinct NULL values will not unify. On the other hand, the search condition IS NULL Field can be represented in XSB as Field = 'NULL'(_)
Using this representation of NULL's the following protocol for queries and updates is established.