Function lemma_usize_clearbit_not_bit_is_set 

pub broadcast proof fn lemma_usize_clearbit_not_bit_is_set(word: usize, b: int)

requires

0 <= b < usize::BITS as int,

ensures

!usize_bit_is_set(word & (!(1usize << (b as usize))), b),

Clearing bit b makes that bit clear.