public void add(AstBase n)
        {
            if (n instanceof AstAbbreviation)
                {
                m_abbreviation = (AstAbbreviation) n;
                }
            else if (n instanceof AstExpression)
                {
                m_expression = (AstExpression) n;
                }
            else
                {
                super.add(n);
                }
        }