Errors on educational example (it's not duplicated).

[daisylab]

While trying to replicate the result from the educational examples, I've got this error.

1. url:
   https://ibm.github.io/LNN/education/examples/reasoning.html#more-complex-reasoning-example

2. code:

from lnn import (Predicate, Variable, Join, And,
                 Exists, Implies, Forall, Model, Fact, World)

model = Model()  # Instantiate a model.
x, y, z, w = map(Variable, ['x', 'y', 'z', 'w'])

# Define and add predicates to the model.
owns = model['owns'] = Predicate('owns', 2)  # binary predicate

3. result:

Traceback (most recent call last):
  File "/home/sungjin/home/lnn-examples/complex.py", line 9, in <module>
    owns = model['owns'] = Predicate('owns', 2)  # binary predicate
TypeError: 'Model' object does not support item assignment