[Hatr828]

SortedSet.ExceptWith can return without removing any elements when called on a view returned by GetViewBetween.
For example:

var set = new SortedSet<int> { 1, 2, 3, 4 };
var view = set.GetViewBetween(2, 3);

view.ExceptWith(view);

// Expected: view is empty;
// Actual: view contains { 2, 3 }.

The method checks the count field directly, but subset views calculate their count lazily. A newly created view can therefore have count == 0 even when it contains elements.

Added regression tests. Fixed the bug by changing count to Count.

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