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