... the buffer is not full, an item may be inserted: ===BufferIn0 [X]============== UpdateBuffer[X] x? : X size The new value is appended to the end of sequence buffer. Extracting an item is possible only if the buffer is not empty; the value obtained is the one at the head of buffer: ===BufferOut0 [X]============= UpdateBuffer[X] x! : X buffer = <> buffer' = tail buffer x! = head buffer These schemas represent partial operations: they describe the effect of a succesful insertion and a succesful extraction, respectively. When the operations are not possible, some messages must be produced. We introduce the type Report ::= ok | full | empty The message ok will be used to indicate a favorable outcome to an operation, full will be used to indicate that the buffer is full, and empty will be used to indicate that the buffer is empty. We include a succesful report defined by the schema: ===Success================== report! : Report report! = ok or one of ...

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