- XEmacs Lisp has two special symbols, @code{nil} and @code{t}, that
-always evaluate to themselves. These symbols cannot be rebound, nor can
-their value cells be changed. An attempt to change the value of
-@code{nil} or @code{t} signals a @code{setting-constant} error.
+In XEmacs Lisp, some symbols always evaluate to themselves: the two
+special symbols @code{nil} and @code{t}, as well as @dfn{keyword
+symbols}, that is, symbols whose name begins with the character
+@samp{@code{:}}. These symbols cannot be rebound, nor can their value
+cells be changed. An attempt to change the value of @code{nil} or
+@code{t} signals a @code{setting-constant} error.