How to model return types that depend on a Header value

I am primarily concerned with defining Apis for third party services, where an answer like "don't do that" does not apply.

My specific situation is where an endpoint returns fairly large type `Foo` with no special headers, but with a `X-Requested-With: XMLHttpRequest` header the return type is shrunk to just the `Bar` type.

It seems as though `Capture` has `Symbol` as it's static equivalent, so `Header` should ideally also have a static equivalent.

Start by examining how Capture provides Symbol as its static equivalent, then compare that design with Header in servant. Done means determining and documenting an appropriate static equivalent for Header that can model return types dependent on the X-Requested-With value.

Written by the indexing model from the issue text.