8.5.5 Keywords
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.
[75] 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. Keyword Type Predicate
(keyword? obj)
Returns #t if obj is a keyword, and  otherwise returns #f. Keyword to String Conversion
(keyword->string keyword)
Returns the name of keyword as a string.
(keyword->string Argentina:)  "Argentina" String to Keyword Conversion
(string->keyword string)
Returns the keyword whose name is string.
(string->keyword "foobar")  foobar: