Function lemma_u8_masked_bit_clear 

pub broadcast proof fn lemma_u8_masked_bit_clear(word: u8, mask: u8, k: int, b: int)

requires

0 <= k <= 8,

k <= b < 8,

mask == (1u8 << (k as usize)) - 1u8,

ensures

!u8_bit_is_set(word & mask, b),

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