Add protection from recursion in sequence elements

[Konard]

Data.Doublets.Sequences/csharp/Platform.Data.Doublets.Sequences/Walkers/LeftSequenceWalker.cs

Lines 106 to 119 in ddc79bd

	protected override IEnumerable<TLink> WalkContents(TLink element)
	{
	var links = _links;
	var parts = links.GetLink(element);
	var start = links.Constants.SourcePart;
	for (var i = parts.Count - 1; i >= start; i--)
	{
	var part = parts[i];
	if (IsElement(part))
	{
	yield return part;
	}
	}
	}

FormatStructure method can be used as an example. The idea is to keep track of visited links.