explicitly add xref dependency
explicitly add xref dependency

muffato/ensembl,james-monkeyshines/ensembl,muffato/ensembl,Ensembl/ensembl,Ensembl/ensembl,muffato/ensembl,james-monkeyshines/ensembl,james-monkeyshines/ensembl,Ensembl/ensembl,james-monkeyshines/ensembl,muffato/ensembl,Ensembl/ensembl