For many years, GHC has implemented an extension to Haskell that allows type
variables to be bound in type signatures and patterns, and to scope over terms.
This extension was never properly specified. We rectify that oversight here.
With the formal specification in hand, the otherwise-labyrinthine path
toward a design for binding type variables in patterns becomes blindingly
clear. We thus extend ScopedTypeVariables to bind type variables
explicitly,
obviating the Proxy workaround to the dustbin of history.