Lurcher Discussions

I found this description of a lurcher in a book called 'the Lurcher' by Frank Sheardown, and thought it to be pretty precise:
'Many years ago greyhounds were greyhounds, whippets were whippets, and any other sort of running dog was a lurcher.
Today things are different, and running dogs are now usually divided between lurchers and longdogs. The author of this book defines these two categories as follows: the longdog is either a pure-bred sighthound (greyhound, whippet, deerhound, wolfhound, borzoi, saluki, Afghan hound, Sloughi, Ibizan Hound, italian greyhound etc) a cross between any of these pure breeds (e.g greyhound crossed with a saluki), or a cross between their crosses (e.g a greyhound/saluki crossed with a greyhound/wolfhound), until such time as blood other than longdog is introduced.
A lurcher is a cross-breed between any longdog and a breed other than longdog.' Traditionally this has usually been collies or terriers because they made the best lurchers, but it can include any breed of dog really.
If you want to get really complicated I've heard that a lurcher bred to a lurcher is also a longdog, and that lurchers are also split into groups depending on how they hunt-some becoming known as 'tumblers.'