Function lemma_u128_masked_bit_keep 

pub broadcast proof fn lemma_u128_masked_bit_keep(word: u128, mask: u128, k: int, b: int)

requires

0 < k <= 128,

0 <= b < k,

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

ensures

u128_bit_is_set(word & mask, b) == u128_bit_is_set(word, b),

Masking with the low-k-bits mask keeps every bit below k unchanged.