Nested list of k-mer counts keyed by
BSgenome.Hsapiens.1000genomes.hs37d5 / BSgenome.Hsapiens.UCSC.hg38 /
BSgenome.Mmusculus.UCSC.mm10, then by exome / transcript / genome,
then by catalog size as a string ("78", "96", "136", "144", "192",
"1536"). Each leaf value is a named integer vector: k-mer → count.
Details
Used to convert counts to density (mutations per megabase of context).
See the planned transform_catalog() function (not yet ported).