-- We do no use List because
-- of the efficiency reason;
-- the order of the derived inverse changes
-- because in GTA we cannot pass any information from
-- children to children.
--
-- Does left-to-right dfs help?

serialize_bintree(x) = sb(x,EOS)
sb(BL,r)      = L(R(r))
sb(BN(x,y),r) = L(sb(x,sb(y,R(r))))