Hint
The general strategy for solving this task is to first make sure
that the first element of the list (the head) is an 'a'. If that
is the case, then you have to deal with the tail of the
list. The tail of a list is again a list and you have to make
sure that it also is a list which contains only a's as elements
(an all_as list) or is empty.
There is also a more declarative way of describing the
predicates that you have to define. Ask yourself: what are
the properties that an all_as list should have?
One possibility is that it is empty. If it is not empty, then
its first element should be an a and the tail should be an
all_as list.