=================================================================
Logtalk - Object oriented extension to Prolog
Release 2.16.0

Copyright (c) 1998-2004 Paulo Moura.  All Rights Reserved.
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| ?- square::nsides(N).

! error(
    existence_error(predicate_declaration, nsides(_)),
    square::nsides(N),
    user)


| ?- q1::(color(Color), side(Side), position(X, Y)).

Color = red
Side = 1
X = 0
Y = 0
yes


| ?- q2::(side(Side), area(Area), perimeter(Perimeter)).

Side = 3
Area = 9
Perimeter = 12
yes
