1st generation message passing systems used an integer "tag" (a.k.a. "type" or "id") to match messages when received
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Most systems allowed wildcard on receive
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wildcard means match any tag i.e. any message
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Unsafe due to unexpected message arrival
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Most could match sender id, some with wildcards
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Wildcards unsafe; strict checks inconvenient
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All systems let users pick the tags
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Unsafe for libraries due to interference
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