Prolog/Modifying the Database

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From the SWI manual: [1]

asserta(+Term)
Assert a fact or clause in the database. Term is asserted as the firsr fact or clause of the corresponding predicate. Equivalent to assert/1, but Term is asserted as first clause or fact of the predicate.

assertz(+Term, -Reference)
Equivalent to asserta/1, asserting the new clause as the last clause of the predicate.

retract(+Term)
When Term is an atom or a term it is unified with the first unifying fact or clause in the database. The fact or clause is removed from the database.

retractall(+Head)
All facts or clauses in the database for which the head unifies with Head are removed. If Head refers to a predicate that is not defined, it is implicitly created as a dynamic predicate. See also dynamic/1.35

Examples:

Let's start with an empty database:

?- human(john).
ERROR: toplevel: Undefined procedure: human/1 (DWIM could not correct goal)

?- assert(human(john)).
true.

?- human(X).
X = john.

?-