Mage-spell casters higher than level 23 and cleric-spell casters
higher than level 13 became less and less likely to cast interesting
spells as level went higher, more and more likely to cast the default
psi-bolt or open wounds. [Level 21 caster had 4.75% chance to cast
touch of death; level 22, 9%; level 23, 13% chance; then level 24, 12.5%;
level 25, 12%; level 30, 10%; level 50 (demon lords/princes), only 6%.]
This oddity in spell selection meant that the Wizard of Yendor gradually
became less likely to use "double trouble" to clone himself as he got
killed off more times and his next incarnation arrived at higher level.
This fix makes high level casters who pick too-high spell usually retry
until they get a valid one instead of just reverting to the default.
Still slightly biased towards psi-bolt and open wounds, since they're
effective even though a bit boring.
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