Function lemma_u16_masked_bit_clear 

pub broadcast proof fn lemma_u16_masked_bit_clear(word: u16, mask: u16, k: int, b: int)

requires

0 <= k <= 16,

k <= b < 16,

mask == (1u16 << (k as usize)) - 1u16,

ensures

!u16_bit_is_set(word & mask, b),

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