public void add(AstBase n)
        {
            if (n instanceof AstSchemaHeader)
                {
                m_schemaHeader = (AstSchemaHeader) n;
                }
            else if (n instanceof AstSchemaExpression)
                {
                m_schemaExpression = (AstSchemaExpression) n;
                }
            else
                {
                super.add(n);
                }
        }