Function lemma_u64_clearbit_not_bit_is_set 

pub broadcast proof fn lemma_u64_clearbit_not_bit_is_set(word: u64, b: int)

requires

0 <= b < 64,

ensures

!u64_bit_is_set(word & (!(1u64 << (b as usize))), b),

Clearing bit b makes that bit clear.