§3.5.(b) Compatibility

A role method bound with the modifier get should have no arguments (it may have arbitrary arguments, which are silently ignored) and should have a return type to which the base field is compatible. A role method returning void will ignore the given value and thus has no effect at all, which will be signaled by a compiler warning.
A role method bound with the modifier set must have a first argument that is compatible to the base field's type (additional arguments - if present - are silently ignored) and must not declare a return type.