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.
A keyword is a single token; therefore no whitespace is allowed between
the identifier and the colon (which is not considered part
of the name of the keyword).
Procedure: keyword? obj
Return #t if obj is a keyword, and otherwise returns #f.
Procedure: keyword->string keyword
Returns the name of keyword as a string.
The name does not include the final #\:.
Procedure: string->keyword string
Returns the keyword whose name is string.
(The string does not include a final #\:.)
