ISO19139 memo - // trick to select MD_DigitalTransferOptions scattered around a record

In current ISO 19139, there are three places to hold URL with authority:
distributionInfo/*/distributor/*/distributorTransferOptions/*/onLine/*/linkage
and distributionInfo/*/transferOptions/*/onLine/*/linkage.

(Darn long path!)

distributorTransferOptions exists for situation in which a dataset has more
than one distributor and each has different linkage.
In other words, it is enough to use transferOptions (a) if there is only one
distributor or (b) if all distributor share the same linkage.

It is painful to prepare for two possibilities. But there is a trick. We
can select //onLine/*/linkage to get entire set of URL's. There is no risk
of selecting MD_DigitalTransferOptions located anywhere because it is only
allowed at these two places.