sub new {
 
    # parameters:
    # [$this],
    # @attributes: tree, messageNum, messageText
    
    my ($this, @attributes) = @_;

    # "process" optional attributes
    my %attributes = (
        tree => undef,
        messageNum => undef,
        messageText => undef,
        @attributes,
    );

    # assert for valid argument list
    if ((keys %attributes) > 3) {
        die "invalid argument list during function call!\n";
    }

    # assert mandatory parameters
    my $tree = $attributes{tree};
    my $messageNum = $attributes{messageNum};
    my $messageText = $attributes{messageText};
    if ((!defined($this)) || (!defined($tree)) || (!defined($messageNum))) {
        die "invalid argument list during function call!\n";
    }