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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 children(self, parent):
 """Return set of children of parent. Parameters parent : :class:`katcp.Sensor` object Parent whose children to return. Returns ------- children : set of :class:`katcp.Sensor` objects The child sensors of parent. """

        if parent not in self._parent_to_children:
            raise ValueError("Parent sensor %r not in tree." % parent)
        return self._parent_to_children[parent].copy()