lib/selection: minor refactorings and comments

Small things I've noticed while reading it all.
- line breaks: I believe <90 is OK, as usually the attempts to reduce
  lengths impair readability
- avoid unnecessary casts; usually the type was visible
  on the same line anyway
- avoid `|` on booleans
- one block gets de-indented (often badly shown in diffs)
- no need for UNRECOVERABLE_ERRORS in a header (and a weird one, too)
- recoverability from failed assertions (in case they're turned off)