This prevents terms of type Bus cap1 a to be coerced to type Bus cap2 b but allows to coerce terms of type Bus cap a to type Bus cap b when Coercible a b holds, hence allowing the zero cost coerce to read and write newtypes from/to a bus for instance. The nominal role ensures a readonly bus cannot be coerced to a readwrite or a writable one.
This prevents terms of type
Bus cap1 ato be coerced to typeBus cap2 bbut allows to coerce terms of typeBus cap ato typeBus cap bwhenCoercible a bholds, hence allowing the zero costcoerceto read and write newtypes from/to a bus for instance. The nominal role ensures a readonly bus cannot be coerced to a readwrite or a writable one.