Function lemma_usize_masked_bit_clear 

pub broadcast proof fn lemma_usize_masked_bit_clear(word: usize, mask: usize, k: int, b: int)

requires

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

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

mask == (1usize << (k as usize)) - 1usize,

ensures

!usize_bit_is_set(word & mask, b),

Masking with the low-k-bits mask clears every bit at or above k.