Function lemma_u16_clearbit_not_bit_is_set 

pub broadcast proof fn lemma_u16_clearbit_not_bit_is_set(word: u16, b: int)

requires

0 <= b < 16,

ensures

!u16_bit_is_set(word & (!(1u16 << (b as usize))), b),

Clearing bit b makes that bit clear.