Get levels of BFS in Prolog

[Agogel]

Hello everybody, i'm new at prolog and to improve my skills i'm trying to make some exercise.
At the moment I'm stuck with a BFS, let assume the tree is something like this:

a
/\
b c
/\
d e

after my query i've something like

X=a;
X=b;
X=c;
X=d;
X=e;

or, at least:

X=a,b,c,d,e;

I'm wondering about how to have results ordered by depth levels as output, something like:

X=[a];
X=[b,c];
X=[d,e];

HEEEEELP

 

[joel76]

First, how do you coded your tree ?