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).
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)"