Iterators can be Independent “from” Their Collections

External iterators pose problems for alias control mechanisms: they have access to collection interals and yet are not accessible from the collection; they may be used in contexts that are unaware of the collection. And yet iterators can benefit from alias control because iterators may fail “unexpectedly” when their collections are modified. We explain a novel aliasing annotation “from” that indicates when a collection intends to delegate its access to internals to a new object and how it can be given semantics using a fractional permission system. We sketch how a static analysis using permissions can statically detect possible concurrent modification exceptions.