Function difficulty_threshold_is_valid 

pub fn difficulty_threshold_is_valid(
    header: &Header,
    network: &Network,
    height: &Height,
    hash: &Hash,
) -> Result<ExpandedDifficulty, BlockError>

Returns Ok(ExpandedDifficulty) if thedifficulty_threshold of header is at least as difficult as the target difficulty limit for network (PoWLimit)

If the header difficulty threshold is invalid, returns an error containing height and hash.