code: def difference_update(self, other, strict=False):
          
          if strict and other not in self:
              raise KeyError(other.difference(self)[0])
          set.difference_update(self, other)
docs: Remove all elements of another set from this RangeSet.
          
          If strict is True, raise KeyError if an element cannot be removed.
          (strict is a RangeSet addition)