System.Xml.Xsl.XsltOld.Compiler.getTextLex C# (CSharp) Method

        private static void getTextLex(string avt, ref int start, StringBuilder lex)
        {
            Debug.Assert(avt.Length != start, "Empty string not supposed here");
            Debug.Assert(lex.Length == 0, "Builder shoud to be reset here");
            int avtLength = avt.Length;
            int i;
            for (i = start; i < avtLength; i++)
            {
                char ch = avt[i];
                if (ch == '{')
                {
                    if (i + 1 < avtLength && avt[i + 1] == '{')
                    { // "{{"
                        i++;
                    }
                    else
                    {
                        break;
                    }
                }
                else if (ch == '}')
                {
                    if (i + 1 < avtLength && avt[i + 1] == '}')
                    { // "}}"
                        i++;
                    }
                    else
                    {
                        throw XsltException.Create(SR.Xslt_SingleRightAvt, avt);
                    }
                }
                lex.Append(ch);
            }
            start = i;
        }