Last modified on 12 April 2012, at 22:07
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From the SWI manual: 
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
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. ?-