...choose 

For notational convenience, we use sometimes the $\sum$ operator to represent a general n-ary (or even infinitary) sum operator.

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...algorithm

We are not interested in analyzing possible attacks to this algorithm, so we do not specify it in detail.

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...operator.

The semantics of the restriction operator on a channel name a ensures that such a channel is ``perfectly'' secure, i.e., no other process outside its scope can communicate through channel a with processes syntactically located inside its scope.

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