In "Clarification for comm.c" John wrote:
> what exactly is the {-1, -1, 0, -1} defining in the struct?
That comes into play in skill.c, where it's used as a factor in
pricing the skills/groups for gain, and as a divisor in practicing.
Which is why the more trains a skill costs, the harder it is to
learn - it's the same number. Yes, 0 means you can't learn that skill,
spells work a bit differently.
It matches the {4, 51, 51, 51} part, which more obviously refers to
levels. Setting something to "over mortal level" also removes it from
the gain list.
