Special proof mode class for adding a modality to a goal.
This used to be done by using `ElimModal` in backwards direction. Having a separate type class for this gets rid of some hacks: - Both `Hint Mode`s in forward and backwards direction for `ElimModal`. - Weird type class precedence hacks to make sure the right instance is picked. These were needed because using `ElimModal` in backwards direction caused ambiguity.
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