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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def pclink(self, parent, child):
 """Create a parent-child relationship."""

    if parent._children is None:
      parent._children = set()
    if child._parents is None:
      child._parents = set()
    parent._children.add(child)
    child._parents.add(parent)