ANSI Common Lisp 6 Iteration 6.1 The LOOP Facility
6.1.9 Notes about LoopTypes can be supplied for loop variables. It is not necessary to supply a type for any variable, but supplying the type can ensure that the variable has a correctly typed initial value, and it can also enable compiler optimizations (depending on the implementation).
The clause repeat n ... is roughly equivalent to a clause such as
(loop for internal-variable downfrom (- n 1) to 0 ...)
but in some implementations, the repeat construct might be more efficient.