% use the directive 'dynamic' to declare a predicate that can be % modified during later program execution

:- dynamic(aa/1).

main(_) :- list_aa, nl, remove_first, list_aa, nl, add_top, list_aa, nl, add_bottom, list_aa.

list_aa :- aa(X), writeq(aa(X)),nl, flush_output, fail ; true.

remove_first :- retract(aa(_)).

add_top :- asserta(aa(300)).

add_bottom :- assertz(aa(xyz)).

aa(1). aa(2). aa(3).