code: def is_anion_cation_bond(valences, ii, jj):
      
      if valences == 'undefined':
          return True
      if valences[ii] == 0 or valences[jj] == 0:
          return True
      return (valences[ii] > 0 > valences[jj]) or (valences[jj] > 0 > valences[ii])
docs: Checks if two given sites are an anion and a cation.
      :param valences: list of site valences
      :param ii: index of a site
      :param jj: index of another site
      :return: True if one site is an anion and the other is a cation (from the valences)