### strengthen u_löb to provide a boxed impl instead of a wand

this makes it slightlymore annoying to use because we have to elliminate the box. one more reason to have a proof mode ;-)

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this makes it slightlymore annoying to use because we have to elliminate the box. one more reason to have a proof mode ;-)

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with
**34 additions**
and
**22 deletions**
**+34**
**-22**

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