<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>*)