Data.Telescope

A telescope is the variable context in a dependently typed program.
Dependent types with free varialbes  are really only well-defined in a telescope.

A segment of a telescope is a tuple of types in that
telescope. These are usually implicit in formal developments, but
seem to be crucial to get a compositional account. Here we use
them to get 'n-ary dependent function' (compare with `Data.Fun`
and `Data.Rel`).

I've learned this from Conor McBride on an SPLV'19 bus ride.
A literary reference would be welcome. This paper is a start:
Unifiers as Equivalences: Proof-Relevant Unification of Dependently Typed Data.
Jesper Cockx, Dominique Devriese, Frank Piessens, ICFP'16.
but they don't seem to have segments and their left action on contexts.