Textual export duplicates "abstract" keyword for OccurrenceUsage

[adaussy]

Exporting the following model from SysON:

package root {
    abstract occurrence test;
    abstract occurrence def Test;
}

Gives:

package root {
    abstract abstract occurrence test;
    abstract occurrence def Test;
}

It should produce instead:

package root {
    abstract occurrence test;
    abstract occurrence def Test;
}