To unify two items simply use `=/2`

, which is defined as if by
the clause

=(X, X).

Please note:Do not confuse this predicate with`=:=/2`

(arithmetic comparison) or`==/2`

(term identity).

Term subsumption is a sort of one-way unification. Term `S` and `T` unify
if they have a common instance, and unification in Prolog instantiates
both terms to that common instance. `S` subsumes `T` if `T` is already an
instance of `S`. For our purposes, `T` is an instance of `S` if there
is a substitution that leaves `T` unchanged and makes `S` identical to `T`.

Subsumption is checked by `subsumes_chk/2`

. It is especially
useful in applications such as theorem provers. The built-in
predicate behaves identically to the original library version but is
much more efficient.

Related predicates are defined in `library(subsumes)`

and
`library(occurs)`

. (For information on these packages see
lib).