The model-theoretic approach is particularly well-understood in the case of the
so called (definite) Horn databases, otherwise known as positive logic
programs, i.e., databases consisting of clauses of the form:
Aj
&A
2
&
...
&A
m
-^C,
with m > 0, and C and
A,
denoting atomic formulas (atoms).
EXAMPLE 1
Suppose that our database DB consists of clauses
Physicist(x) -^Good_mathematician(x)
Physicist(Einstein)
Businessman(Iacocca).
This database has several different models, the largest of which is the model
Af
max
in which both men are at the same time businessmen
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