Function decode_kept_fields 

pub fn decode_kept_fields(
    encoded: &[u8],
    document_type: DocumentTypeRef<'_>,
) -> Result<BTreeMap<String, Value>, ProtocolError>

Reads the values encode_kept_fields wrote, under document_type: each kept path to its value, typed as reading the document gives it, a path the document held no value at left out. Refuses bytes that end before the last path or run past it.