* Returns null or the first sequence in the dataset which is identical to
* xref.mapTo, and has a) a primary dbref matching xref, or if none found, the
* first one with an ID source|xrefacc
* Returns null or the first sequence in the dataset which is identical to
* xref.mapTo, and has a) a primary dbref matching xref, or if none found, the
* first one with an ID source|xrefacc