
222 Topor and Sonenberg
in language L. Then M(D,L) is {p(a)} if a is the only constant in L but
M(D,L) is {p(a)
9
q(b)} if a and b are the constants in L. •
The model M(D,L) was defined implicitly for stratified general databases by
Chandra and Harel [1985] and explicitly for stratified general programs by Apt,
Blair, and Walker [1988] where it was denoted by M
D
. In each case, they used
for L the language of D. The use of the word "standard" is motivated by a
result in Apt et al. [1988] that states M(D,L) is independent of the level map-
ping used to show D is stratified, by Lifschitz's demonstration that M(D,L)
arises naturally in an attemp