Function lemma_u16_masked_bit_keep 

pub broadcast proof fn lemma_u16_masked_bit_keep(word: u16, mask: u16, k: int, b: int)

requires

0 < k <= 16,

0 <= b < k,

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

ensures

u16_bit_is_set(word & mask, b) == u16_bit_is_set(word, b),

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