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### 2.7.15 Exception Handling

In quite a few cases, the properties given for the operator up to this point do not imply any particular result. A good example would be an expression such as [[`left`]] `nil`, which appears to represent the left side of an empty pair. It can be argued that expressions like this have no sensible interpretation and should never be used, so it would be appropriate to leave them undefined. On the other hand, attempts to evaluate such expressions occur frequently by mistake, and in any case, the virtual machine emulator should be designed to do something reasonable about them if only for the sake of reporting the error. The chosen remedy for this situation addresses the need for error reporting, and also turns out to be useful in other ways.

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