code: def split_top_down(lower, upper, __fval=None, **fval):
      
      if __fval is not None:
          fval.update(__fval)
      if lower in upper._overload_fields:
          ofval = upper._overload_fields[lower]
          if any(k not in ofval or ofval[k] != v for k, v in six.iteritems(fval)):  # noqa: E501
              return
          upper._overload_fields = upper._overload_fields.copy()
          del(upper._overload_fields[lower])
docs: This call un-links an association that was made using bind_top_down.
      Have a look at help(bind_top_down)