aExpr = nsnull:
aExpr = nsnull;
getExprAttr(txStylesheetAttr* aAttributes,            PRInt32 aAttrCount,            nsIAtom* aName,            PRBool aRequired,            txStylesheetCompilerState& aState,            nsAutoPtr<Expr>& aExpr){    aExpr = nsnull;    txStylesheetAttr* attr = nsnull;    nsresult rv = getStyleAttr(aAttributes, aAttrCount, kNameSpaceID_None,                               aName, aRequired, &attr);    if (!attr) {        return rv;    }    rv = txExprParser::createExpr(attr->mValue, &aState,                                  getter_Transfers(aExpr));    if (NS_FAILED(rv) && !aRequired && aState.fcp()) {        // use default value in fcp for not required exprs        aExpr = nsnull:        return NS_OK;    }    return rv;}