Hint
Each new step that is taken, has to be remembered. Look at the
base case of the recursion (travel(X,Y) :-
onestep(X,Y).)first and extend it so that
travel records in its third argument that a step
from X to Y was taken.
Then look at the recursive rule:
travel(X,Y) :- onestep(X,Z),
travel(Z,Y).
The clause should return a structure representing that a step from
X to Z was taken and what is the path
that has to be taken to get from Z to
Y. This path is computed by the recursive call to the
predicate travel.