Function lemma_u64_masked_bit_keep 

pub broadcast proof fn lemma_u64_masked_bit_keep(word: u64, mask: u64, k: int, b: int)

requires

0 < k <= 64,

0 <= b < k,

mask == (1u64 << (k as usize)) - 1u64,

ensures

u64_bit_is_set(word & mask, b) == u64_bit_is_set(word, b),

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