Specifically, the line type Out = Url should be type Output = Url now. Also, that code block doesn't compile when you paste it into a file (for starters, it's missing a curly brace). It would be nice if you updated the docs to reflect the current code base and make the example work out of the box.
The first example given on this page is out of date: https://propensive.com/opensource/contextual/#a-simple-example
Specifically, the line
type Out = Urlshould betype Output = Urlnow. Also, that code block doesn't compile when you paste it into a file (for starters, it's missing a curly brace). It would be nice if you updated the docs to reflect the current code base and make the example work out of the box.