naaaaaaah, that is priest feature only...then again, casting with low sp COULD
be allowed with penalty. That is, casting 170sp spell with less than 170sp is
allowed, but you lose whole 170 (going below zero in mana) and _additionally_
lose, say, half of HP and EP. Sounds fair eh, you can squeeze that last spell,
with a cost.

COULD 170sp is _additionally_ spell, That sounds sensible. However, you could also add some special flavour to
casting with insufficient sp. Something like losing (temporarily) some
stats (con, str), or going stunned for a few rounds from the effort. This
has been (quite successfully) used in a roguelike game Angband.

This way this would be used as a last resort in a panic situation and not
in a regular use for exp monster killing. Because of the nature of this it
would require some set_some_flag to be able to do this, so that you
don\'t get stunned by accident