ANSI Common Lisp 6 Iteration 6.1 The LOOP Facility 6.1.1 Overview of the Loop Facility
188.8.131.52 Loop Keywords
Loop keywords are not true keywords1;
they are special symbols, recognized by name rather than object identity,
that are meaningful only to the loop facility.
A loop keyword is a symbol but is recognized by its name
(not its identity), regardless of the packages in which it is accessible.
In general, loop keywords are not external symbols of the common-lisp package,
except in the coincidental situation that a symbol with the same name as a
loop keyword was needed for some other purpose in Common Lisp. For example,
there is a symbol in the common-lisp package whose name is "UNLESS" but
not one whose name is "UNTIL".
If no loop keywords are supplied in a loop form,
the Loop Facility executes the loop body repeatedly; see Section 184.108.40.206.1 Simple Loop.