efl/src/tests/eolian/data/error.eo

error Foo = "something bad happened"; [[Error doc. @since 1.66]]
error @beta Bar = "another bad thing happened"; [[Another error doc]]

class @beta Error {
   methods {
      foo {
         return: error(Foo);
      }
      bar {
        return: error(Foo, Bar);
      }
   }
}