PyDatalog: list of values in answer

Question

In PyDatalog I have defined the following assertions:

#stations
assert_fact('station', 'A' ,'yellow') 
assert_fact('station', 'B' ,'yellow')
assert_fact('station', 'C' ,'yellow')
assert_fact('station', 'D' ,'yellow')
#sections
assert_fact('stretch', 'A' ,'B')
assert_fact('stretch', 'B' ,'C')
assert_fact('stretch', 'C', 'D')

And I would like to ask the database if there is a way to get to D from A. The following code should work, because it answers set([()]) if there is a way, and None if there isn't. But it doesn't give me the result of the different evaluations of Z. I would like to know also the route, for example: A B C D

load("""
route(X,Y) <= stretch(X,Y)
route(X,Y) <= stretch(X,Z) & route(Z,Y)
""")

I have tried with unbound values, but it only gives me the result of the first iteration:

load("""
route(X,Y,P) <= stretch(X,Y) & (P==Y)
route(X,Y,P) <= stretch(X,P) & route(P,Y,Z)  
""")

I think that the problem is that it only takes P in the first iteration. Or should I use aggregation functions? I don't understand very well how I could use concat...

Thanks in advance.