The for and as constructs provide a termination test
that is determined by the iteration control clause.
The repeat construct causes termination after a specified
number of iterations.
(It uses an internal variable to keep track of the number of iterations.)
The while construct takes one form, a test,
and terminates the iteration if the test evaluates to false.
A while clause is equivalent to the expression
(if (not test) (loop-finish)).
The until construct is the inverse of while;
it terminates the iteration if the test evaluates to
any non-nil value.
An until clause is equivalent to the expression
(if test (loop-finish)).
The always construct takes one form and
terminates the loop if the form ever evaluates to false;
in this case, the loop form returns nil.
Otherwise, it provides a default return value of t.
The never construct takes one form and
terminates the loop if the form ever evaluates to true;
in this case, the loop form returns nil.
Otherwise, it provides a default return value of t.
The thereis construct takes one form and
terminates the loop if the form ever evaluates to
a non-nil object;
in this case, the loop form returns that object.
Otherwise, it provides a default return value of nil.
If multiple termination test clauses are specified, the loop form terminates if any are satisfied.
For more information, see Termination Test Clauses.