message: Add definition of opposite category
diff: @@ -48,6 +48,13 @@ record {
  id = id,
  seq = compose,
  },
  +
  + category-opposite = fun c => record {
  + Object = c.Object,
  + Arrow = fun A B => c.Arrow B A,
  + id = c.id,
  + seq = fun A B C a-b b-c => c.seq C B A b-c a-b,
  + },
  } : Record {
  ||| The polymorphic identity function.
  id : Fun (A : Type) -> A -> A,
  @@ -139,4 +146,7 @@ record {
  ||| Category of Pikelet functions and types.
  category-pikelet : Category,
  +
  + ||| Opposite categories.
  + category-opposite : Category -> Category,
  }
  