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A db-spec has the form of a SpecList:
= [spec1, …, specM]
= functor(argspec1, …, argspecN)
= + | -
where functor is a Prolog atom. The case N = 0 is allowed.
A spec F(argspec1, …, argspecN) is
applicable to any nonvar term with principal functor
F/N.
When storing a term T, we generate a hash code for every
applicable spec in the db-spec, and a reference to T is stored
with each of them. (More precisely, with each element of the set of
generated hash codes). If T contains nonvar elements on each
+ position in the spec, then the hash code depends on each of
these elements. If T does contain some variables on
+ position, then the hash code depends only on the functor
of T.
When fetching a term Q, we look for an applicable spec for
which there are no variables in Q on positions marked
+. If no applicable spec can be found, a domain error is raised.
If no spec can be found where on each + position a nonvar
term occurs in Q, an instantiation error is raised.
Otherwise, we choose the spec with the most + positions,
breaking ties by choosing the leftmost one.
The terms that contain nonvar terms on every +
position will be looked up using indexing based on the principal
functor of the term and the principal functor of
terms on + positions. The other (more general)
terms will be looked up using an indexing based on the principal
functor of the term only.