clean up of unmapped objects no longer an option but enforced as external_db_id also added to the delete line to ensure safety

Allow specification of up to 3 separate databases: one to read transcripts from, one to read oligo features from, oe to write xrefs to.

By default, check if there are existing xrefs (for all or user-specified arrays) and exit if there are. Only delete if -delete is explicitly specified.

Check for situations where someone has used spaces to separate the fields of the file rather than tabs.

Allow comments in files.

Skip whitespace-only or blank lines.