17     FuncRef { null: bool },
18     ExternRef { null: bool },
19     AnyRef { null: bool },
20     ExnRef { null: bool },
21     ContRef { null: bool },
190             FuncRef => DiffValue::FuncRef { null: true },  in arbitrary_of_type()
191             ExternRef => DiffValue::ExternRef { null: true },  in arbitrary_of_type()
192             AnyRef => DiffValue::AnyRef { null: true },  in arbitrary_of_type()
193             ExnRef => DiffValue::ExnRef { null: true },  in arbitrary_of_type()
194             ContRef => DiffValue::ContRef { null: true },  in arbitrary_of_type()
238             DiffValue::ExternRef { null } => null.hash(state),  in hash()
239             DiffValue::FuncRef { null } => null.hash(state),  in hash()
240             DiffValue::AnyRef { null } => null.hash(state),  in hash()
241             DiffValue::ExnRef { null } => null.hash(state),  in hash()
242             DiffValue::ContRef { null } => null.hash(state),  in hash()
276             (Self::FuncRef { null: a }, Self::FuncRef { null: b }) => a == b,  in eq()
277             (Self::ExternRef { null: a }, Self::ExternRef { null: b }) => a == b,  in eq()
278             (Self::AnyRef { null: a }, Self::AnyRef { null: b }) => a == b,  in eq()
279             (Self::ExnRef { null: a }, Self::ExnRef { null: b }) => a == b,  in eq()
280             (Self::ContRef { null: a }, Self::ContRef { null: b }) => a == b,  in eq()