In the for-as-on-list subclause, the for or as construct iterates over a list. It checks for the end of the list as if by using atom. The variable var is bound to the successive tails of the list in form1. At the end of each iteration, the function step-fun is applied to the list; the default value for step-fun is cdr. The loop keywords on and by serve as valid prepositions in this syntax. The for or as construct causes termination when the end of the list is reached.