Proof of True_implies_equals is stored with "open" derivation to
facilitate simplification of proof terms.
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