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.