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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 _add_node(self, idx, unique_idx, frac_coords):
 """ Add information about a node describing a critical point. :param idx: unique index :param unique_idx: index of unique CriticalPoint, used to look up more information of point (field etc.) :param frac_coord: fractional co-ordinates of point :return: """

        self.nodes[idx] = {'unique_idx': unique_idx, 'frac_coords': frac_coords}