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• All operators that do not reside within a loop contain the default StreamScope.

• The two main operations of a StreamScope are nest() and unnest()

• Every invocation of the DataStream iterate operation generates a new StreamScope, nested on its outer scope.

• The level of each StreamScope (getLevel()) is the number of its outer scopes. Fig.2 shows the assigned scope levels on each operator in an example graph.

• StreamScopes can be compared topologically (isInnerOf(..), isOuterOf()).

• Operators who reside in the exact same loop should have equal StreamScope

• All outer scopes of a StreamScope can be retrieved

• (e.g. scope.getContextForLevel(scope.getLevel()-2), or  scope.unest().unest())

II - Cyclic Flow Control and Physical Implementation of Feedback Channels

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