Function leaves 

pub fn leaves(r: RegLan) -> impl Iterator<Item = RegLan>

Iterator that enumerates the leaves of r

• A leaf is an atomic sub-term of r (i.e., a term t such that f.expr is either BaseRegLan::Empty, or BaseRegLan::Epsilon or BaseRegLan::Range)
• All leaves are listed once (no duplicates)
• If r itself is atomic, the iterator just produces r and nothing else.