canonical 'cases'/'induct' rules for n-tuples (n=3..7)
(really belongs to theory Product_Type, but doesn't work there yet)
Last = List +
consts last :: 'a list => 'a
recdef last "measure (%xs. length xs)"
"last [x] = x"
"last (x#y#zs) = last (y#zs)"
end