pair :: Grammar p (b :- a :- t) ((a, b) :- t)
pair = Iso
  (\(b :- a :- t) -> (a, b) :- t)
  (\((a, b) :- t) -> b :- a :- t)
pair :: Grammar p (b :- a :- t) ((a, b) :- t)
pair = Iso
  (\(b :- a :- t) -> (a, b) :- t)
  (\((a, b) :- t) -> b :- a :- t)
pair = Iso
  (\(b :- a :- t) -> (a, b) :- t)
  (\((a, b) :- t) -> b :- a :- t)