Keywords are similar to symbols.
The main difference is that keywords are self-evaluating
and therefore do not need to be quoted in expressions.
They are used mainly
for specifying keyword arguments.
 keyword = identifier:
A keyword is a single token; therefore, no whitespace is allowed
between the identifier and the :.
The : is not considered part of the name of the keyword.
126.96.36.199 Keyword Type Predicate
Returns #t if obj is a keyword, and otherwise returns
188.8.131.52 Keyword to String Conversion
Returns the name of keyword as a string.
(keyword->string Argentina:) "Argentina"
184.108.40.206 String to Keyword Conversion
Returns the keyword whose name is string.
(string->keyword "foobar") foobar: