We first define for each node n of c the set N n consisting of all nodes higher than n, all nodes of the same depth as n but preceding (on the left), and n. Now for each N n with |N n | = i let the function ψ i be the restriction of ψ to N n (ψ|N n ).
then we can show by induction for each the equivalence is established by constructing a minimal model M i also on the universe U of M. the isomorphism between M|Σ and M i |Σ is the identity function.