expand_proof now also takes an optional term describing the proposition
of the theorem to be expanded (to avoid problems with different theorems
having the same names).
Sep1 = Main +
consts sep :: "'a * 'a list => 'a list"
recdef sep "measure (%(a,xs). length xs)"
"sep(a, x#y#zs) = x # a # sep(a,y#zs)"
"sep(a, xs) = xs"
end