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 section Machine Dependent Features.
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). See section Symbols.