Function lookup 

pub fn lookup<'a, E, I, S>(entity: &'a E, path: I) -> Option<Cow<'a, Value>>
where
    E: EntityFields + ?Sized,
    I: IntoIterator<Item = S>,
    S: AsRef<str>,

The value at path, field names from the top of entity; the whole entity for an empty path. None as soon as a step is missing.