ByteWeight

here `mem` parameter is actually unused. When we add the corresponding command line option (and why we haven't added it yet?) this will provoke an unused parameter warning. 

In OCaml we have convention (respected by the compiler) that unused parameters are either prefixed with underscore, like `_mem` or just substituted with `_`. I prefer the former, since it explains _what_ particular parameter you're not using. Also, you can also annotate it with type, so in this case something like `(_ : mem)` will be very fine.

Also, since you're not using this mem parameter it is better to use `Table.iter` it is much faster.