class Point {
    has ( $x, $y ) :reader :writer :new :isa(Int);

    method clear () {
        ( $x, $y ) = ( 0, 0 );
    }
}

class Point3D isa Point {
    has $z :reader :writer :new :isa(Int);

    method clear () {
        $self->next::method;
        $z = 0;
    }
}