Backmasked diffs currently explicitly list every position where they are masked, instead of making use of the third column. For instance:

>sample X
- 160 5

Would become:

>[BM] sample X
-	160	1
-	161	1
-	162	1
-	163	1
-	164	1

Some of our data has thousands of positions masked at in any given place, so this inefficiency could be a real problem. Prioritizing this will depend on whether we intend on actually using backmasking to hide positions, or if it will still instead be done using matUtils.