https://stackoverflow.com/questions/25713668/do-a-covariant-filter-on-an-hlist
https://stackoverflow.com/questions/25713668/do-a-covariant-filter-on-an-hlist
TODO: create different type of librarians/traversers, e.g.
TODO: create different type of librarians/traversers, e.g. no need to keep path if the traversal does not have a path-step
Type Class witnessing that an 'HList' can be spanned with a 'Poly' to produce two 'HList's
Type Class witnessing that an 'HList' can be spanned with a 'Poly' to produce two 'HList's
TODO: try to convert End to shapeless.Coproduct