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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 append(self, species, coords, validate_proximity=True, properties=None):
 """ Appends a site to the molecule. Args: species: Species of inserted site coords: Coordinates of inserted site validate_proximity (bool):
 Whether to check if inserted site is too close to an existing site. Defaults to True. properties (dict):
 A dict of properties for the Site. Returns: New molecule with inserted site. """

        return self.insert(len(self), species, coords,
                           validate_proximity=validate_proximity,
                           properties=properties)