#### 7.1.2 External representations

<Datum> is what the `read` procedure (section see Input) successfully parses. Note that any string that parses as an <expression> will also parse as a <datum>.

    <datum> –> <simple datum> | <compound datum>
    <simple datum> –> <boolean> | <number>
         | <character> | <string> |  <symbol>
    <symbol> –> <identifier>
    <compound datum> –> <list> | <vector>
    <list> –> (<datum>*) | (<datum>+ . <datum>)
           | <abbreviation>
    <abbreviation> –> <abbrev prefix> <datum>
    <abbrev prefix> –> ' | ` | , | ,@
    <vector> –> #(<datum>*)