#### §3.1.(e) Callout override

If an inherited role method is concrete, callout binding regarding this method must use the token "`=>`" instead of "`->`" in order to declare that this binding overrides an existing implementation.
Using the "`=>`" operator for an abstract method is an error.
It is also an error (and not useful anyway) to callout-bind a method that is implemented in the same class as the binding.