@Override
  protected boolean parseExpr(PsiBuilder builder) {
    final PsiBuilder.Marker seq = builder.mark();
    final boolean b = parseExprSingle(builder);
    if (b && builder.getTokenType() == XPathTokenTypes.COMMA) {
      do {
        builder.advanceLexer();
        if (!parseExprSingle(builder)) {
          builder.error(XPathBundle.message("parsing.error.expression.expected"));
        }
      } while (builder.getTokenType() == XPathTokenTypes.COMMA);
      seq.done(XPath2ElementTypes.SEQUENCE);
    } else {
      seq.drop();
    }
    return b;
  }
@Override
  protected boolean parseExpr(PsiBuilder builder)