private static string ToStringUnsafe <T>(this ITreeNode <T> root, StringFormat sf)
        {
            switch (sf)
            {
            case StringFormat.Default:
                return($"{root.GetType()}: cur: {root.Current}, deep: [{root.ChildrenToString(sf, ", ")}]");

            case StringFormat.NewLine:
            {
                string[] lines = $"{root.Current}{(root.Count > 0 ? ":" : "")}\n<NEEDTAB_TreeNode>{root.ChildrenToString(sf, "\n")}\n</NEEDTAB_TreeNode>".Split(new char[] { '\n' }, StringSplitOptions.RemoveEmptyEntries);
                int      count = 0, countOpen, countClose;
                Regex    regOpen  = new Regex("<NEEDTAB_TreeNode>");
                Regex    regClose = new Regex("</NEEDTAB_TreeNode>");
                for (long i = 0; i < lines.LongLength; i++)
                {
                    countOpen  = regOpen.Matches(lines[i]).Count;
                    countClose = regClose.Matches(lines[i]).Count;
                    count     += countOpen - countClose;
                    if (count > 0)
                    {
                        lines[i] = lines[i].Insert(0, new string('\t', count));
                    }
                    if (countOpen != 0)
                    {
                        lines[i] = lines[i].Replace("<NEEDTAB_TreeNode>", "");
                    }
                    if (countClose != 0)
                    {
                        lines[i] = lines[i].Replace("</NEEDTAB_TreeNode>", "");
                    }
                }
                Regex         needRemove        = new Regex("^[\\t| ]+$");
                List <string> LinesWithoutSpace = new List <string>(lines);
                LinesWithoutSpace.RemoveAll(str => needRemove.Match(str).Success || str.Length == 0);
                return(string.Join("\n", LinesWithoutSpace));
            }

            case StringFormat.Base:
                return(root.GetType().ToString());

            default:
                throw new NotSupportedException($"Формат {sf} не поддерживается.");
            }
        }