instance_bug.m:013: Error: non-abstract instance declaration in module
instance_bug.m:013:   interface.
instance_bug.m:013:   If you intend to export this instance, move this
instance_bug.m:013:   declaration to the implementation section, replacing it
instance_bug.m:013:   in the interface section with its abstract version, which
instance_bug.m:013:   omits the `where [...]' part.