the type placeholder `_` is not allowed within types on item
signatures

// The type parameters are:
// O: this Router's data type
// B: The parent router's data type
pub struct DataRouter<'a, O, B = O> where O: 'a, B: 'a {
    parent: Option<&'a DataRouter<'a, B, _>>, // Here the problem `_`
    range: Option<Range<usize>>,
    data: Option<O>
}