def transition_system(bdd):
    """Return the transition relation of a graph."""
    dvars = ['x0', "x0'", 'x1', "x1'"]
    for var in dvars:
        bdd.add_var(var)
    s = (
        "((!x0 & !x1) -> ( (!x0' & !x1') | (x0' & !x1') )) & ((x0 & !x1) -> ! (x0' & x1')) & ((!x0 & x1) -> ( (!x0' & !x1') | (x0' & x1') )) & ! (x0 & x1)"
        )
    transitions = bdd.add_expr(s)
    return transitions
