Function lemma_u64_masked_bit_clear 

pub broadcast proof fn lemma_u64_masked_bit_clear(word: u64, mask: u64, k: int, b: int)

requires

0 <= k <= 64,

k <= b < 64,

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

ensures

!u64_bit_is_set(word & mask, b),

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