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MessageEnd (from BasicInteractions ) on page 515
A Gate is a connection point for relating a Message outside an InteractionFragment with a Message inside the InteractionFragment .
Gate is a specialization of MessageEnd .
Gate s are connected through Messages. A Gate is actually a representative of an OccurrenceSpecification that is not in the
same scope as the Gate .
Gate s play different roles: we have formal gates on Interactions, actual gates on InteractionUse s, expression gates on CombinedFragment s.
 The message leading to/from an actualGate of an InteractionUse must correspond to the message leading from/to the formalGate
with the same name of the Interaction referenced by the InteractionUse .
 The message leading to/from an (expression) Gate within a CombinedFragment must correspond to the message leading from/to
the CombinedFragment on its outside.
The gates are named either explicitly or implicitly. Gate s may be identified either by name (if specified), or by a constructed
identifier formed by concatenating the direction of the message and the message name (e.g., out_CardOut). The gates and the
messages between gates have one purpose, namely to establish the concrete sender and receiver for every message.
Gate s are just points on the frame, the ends of the messages. They
may have an explicit name (see Figure 14.19).
The same gate may appear several times in the same or different diagrams.