We present a bisimulation method for proving the contextual
equivalence of packages in lambda-calculus with full existential and
recursive types.  Unlike traditional logical relations (either
semantic or syntactic), our development is "elementary," using only
sets and relations and avoiding advanced machinery such as domain
theory, admissibility, and TT-closure.  Unlike other bisimulations,
ours is complete even for existential types.  The key idea is to
consider _sets_ of relations---instead of just relations---as
bisimulations.