When having a look at http://isabelle.in.tum.de/repos/isabelle/rev/39fe503602fb
it came to my mind that this might be implemented more generally, e.g.
the_aux :: 'a list => 'a
the_aux [x] = x
the :: 'a list => 'a
the xs = the_aux (remdups xs)
which would yield a Match exception without any low-level tinkering.
(of course, the situation is more complicated with enum and such, but
this example illustrated the principle: native pattern match failure to
express non-uniqueness).
Florian
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