code: def append(self, species, coords, validate_proximity=True, properties=None):
          
          return self.insert(len(self), species, coords,
                             validate_proximity=validate_proximity,
                             properties=properties)
docs: 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.