6.21: adda/lexicon/$symbol can indicate
literal symbol just like quotes mean literal string:
. there are 3 categories of symbols:
# names composed of digits are literal values of type number;
# names preceded by double-quote are literal values of type character string;
# names preceded by single-quote are unevaled symbols
(references to a symbol that was previously declared);
# names preceded by ($) are declaring literal values of type symbol;
(it's not a variable or constant representing some other value).
. in english we would tend to put a word in quotes to mean this,
but in a programming language, we want to reserve quotes for
values of type character string .
. this use for $ was first mentioned as
a namespace for value literals .]