α( 〈 items, limit 〉 ) = 〈 mk_set( items ), limit 〉 where
mk_set( ε ) = ∅
mk_set( e ·s ) = mk_set(s) \uplus {e}
ρ( push ) = put, ρ( pop ) = get
ξ(s.settop(i)) = s.pop(); s.push(i)
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Behavioral subtypes -- correspondence