code: def compute_equiv_class(atom):
          
      try:
          equiv_class = atom.number + \
                        1000*(atom.charge+10) + \
                        100000*(atom.hcount) + \
                        1000000*(atom.weight)
      except TypeError:
          raise ValueError, \
                "Can't compute number from atom.number %s atom.charge %s atom.hcount %s"\
                " atom.weight %s"%(atom.number, atom.charge, atom.hcount, atom.weight)
      return equiv_class
docs: (atom)->Computes a unique integer for an atom