hexirp
知らなかった…… 型注釈の構文を流用しているからかな
As with other type signatures, you can give a single signature for several data constructors. In this example we give a single signature for `T1` and `T2`:https://ghc.gitlab.haskell.org/ghc/doc/users_guide/exts/gadt_syntax.html#gadt-syntax-odds-and-ends