IEnumerable<T>.Diff performs comparisons up to four times per pair

[Phill]

The method uses a multicast followed by a comparison in each stream that performs the filtering. The comparison should be done prior to the multicast and injected into the multicast functions so that it is only performed once per pair of inputs.

[Phill (phill.l)]

Upon checking the comparison is performed at most twice for the different and same multicast targets, usually the 'same' multicast target is not implemented that therefore never called.