some issues in DvOrdinal

[Sergio Miranda Freire]

Is it necessary to test the invariant below in the constructor?
     if symbol is None or isinstance(symbol,DvCodedText) == False:
            raise ValueError(_("Invalid DvOrdinal.symbol"))
        else:
            self.symbol=symbol

The invariant below is not enforced:
Limits_valid: limits /= Void and then limits.meaning.is_equal(“limits”)
Reference_range_valid: other_reference_ranges /= Void and then other_reference_ranges.has(limits)

and then the method limits is not correctly implemented: it should have no arguments and should return a ReferenceRange object corresponding to an element of otherReferenceRanges attribute whose meaning is "limits".

The method compare (<) is not implemented:
it should have a DvOrdinal argument and return 1, 0 and -1 according to the comparisons of the corresponding value attributes.

The method isStrictlyComparableTo is not correclty implemented:
In java, they returned true if the symbols came from the same terminology. Maybe this is the maximum we can get without more information.