### Method callbacks

One common pattern in the actor programming model is to carry the result of
a method call as parameter of another one. This is called callback, and can
be used in many contexts to avoid blocking the main application process
while waiting for results.

Example:
```python
import uactor

class ActorA(uactor.Actor):

    def send(self, target):
        return target('ping')

class ActorB(uactor.Actor):

    def receive(self, value):
        return 'pong' if value == 'ping' else 'error'

actor_a = ActorA()
actor_b = ActorB()
print(actor_a.send(actor_b.receive))
# pong
```