Producing all allocations by n items from a list
(defun allocations (source length)
  (if (= 1 length)
      (list (list (car source)))
      (loop for processed = nil then (cons (car i) processed)
         for i on source
         for todo = (cdr i)
         appending
           (loop for intermediate
              in (allocations
                  (append (reverse processed) todo)
                  (1- length))
              appending
                (loop for prefix = nil then (cons (car suffix) prefix)
                   for suffix on intermediate
                   collect (append prefix (list (car i)) suffix))))))


This should be a more general case of the problem (i.e. it makes no assumption of the nature of the elements in the list), but it doesn't verify that there is enough of the elements in the source list to build the required number of permutations.