<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _bdd(node):
 """Return a unique BDD."""

    try:
        bdd = _BDDS[node]
    except KeyError:
        bdd = _BDDS[node] = BinaryDecisionDiagram(node)
    return bdd