Function page_content_hash 

pub fn page_content_hash(bytes: &[u8]) -> CapHash

Content hash of a single page: the SSZ hash_tree_root of the page as a ByteVector[PAGE_SIZE] (zero-padded), under the cap digest (SHA-256). This is the value a materialized page contributes to the cap merkle and the precomputed PageBytes::hash kept by the substitution invariant.