public INode FindOldestItemOnBranch(IEnumerable <INode> nodes)
{
Node[] nodesArray = nodes.Select(Unwrap).ToArray();
// Make sure these nodes are from the same branch.
IBranch sourceBranch = nodesArray.Select(node => _containedInBranch[node]).Distinct().SingleOrDefault();
if (sourceBranch == null)
{
return(null);
}
OrderedSet <Node> set = _branches[sourceBranch];
INode oldest = set.PickFirst(nodesArray);
return(oldest);
}