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).
Sep2 = Main +
consts sep :: 'a list => 'a => 'a list
recdef sep "measure length"
"sep (x#y#zs) = (%a. x # a # sep zs a)"
"sep xs = (%a. xs)"
end