Some simple functions can't be satisfied ?

[vsraptor]

Very often I get :

 In [137]: fa[1]
 Out[137]: Or(And(x, q), w)

 In [138]: with ~q,~w : print("00> %s" % fa[1].satisfy_one())
 ---------------------------------------------------------------------------
 AttributeError                            Traceback (most recent call last)
 <ipython-input-138-b3b11b8a9e9c> in <module>()
 ----> 1 with ~q,~w : print("00> %s" % fa[1].satisfy_one())

 /usr/local/lib/python3.6/dist-packages/pyeda/boolalg/expr.py in satisfy_one(self)
743                 aupnt = _assume2point()
744                 soln = _backtrack(self.restrict(aupnt))
 --> 745                 soln.update(aupnt)
746                 return soln
747             else:

 AttributeError: 'NoneType' object has no attribute 'update'

---

 In [139]: fa[5]
 Out[139]: Xor(Or(x, q), w)

 In [140]: with ~q,~w : print("00> %s" % fa[5].satisfy_one())
 00> {x: 1, w: 0, q: 0}