§4.2.(d) Callin methods

Role methods to be bound by a callin replacement binding must have the modifier callin. This modifier is only allowed for methods of a role class.
A method with the callin modifier can only be called

• via a callin replace binding
• by a super or tsuper call from an overriding callin method.

It is illegal for a callin method

• to be called directly,
• to be bound using a callout binding, and
• to be bound to a base method using a before or after callin binding.

Despite these rules a second level role — which is played by the current role — can intercept the execution of a callin method using any form of callin binding.

A callin method cannot override a regular method and vice versa, however, overriding one callin method with another callin method is legal and dynamic binding applies to callin method just like regular methods.
A callin method must not declare its visibility using any of the modifiers public, protected or private. Since callin methods can only be invoked via callin bindings such visibility control would not be useful.