Function lemma_u128_masked_bit_clear 

pub broadcast proof fn lemma_u128_masked_bit_clear(word: u128, mask: u128, k: int, b: int)

requires

0 <= k <= 128,

k <= b < 128,

mask == (1u128 << (k as usize)) - 1u128,

ensures

!u128_bit_is_set(word & mask, b),

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