def prior_sample(bn):
    """Randomly sample from bn's full joint distribution. The result
    is a {variable: value} dict. [Fig. 14.13]"""
    event = {}
    for node in bn.nodes:
        event[node.variable] = node.sample(event)
    return event
Randomly sample from bn's full joint distribution. The result
    is a {variable: value} dict. [Fig. 14.13]