Symbols
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A "symbol" is one or more characters chosen from the set of all
letters (both upper and lower case), digits and the three characters
`_.$'. On most machines, you can also use `$' in symbol names;
exceptions are noted in Note:Machine Dependencies. No symbol may
begin with a digit. Case is significant. There is no length limit:
all characters are significant. Symbols are delimited by characters
not in that set, or by the beginning of a file (since the source
program must end with a newline, the end of a file is not a possible
symbol delimiter). Note:Symbols.