-- = (\a1..an \x1..xn. [])
| WpCast Coercion -- A cast: [] `cast` co
- -- Guaranteedn not the identity coercion
+ -- Guaranteed not the identity coercion
| WpApp Var -- [] d the 'd' is a type-class dictionary
| WpTyApp Type -- [] t the 't' is a type or corecion