// note
// self.fishes => Vec<Box<fish::Fish>>

for f in self.fishes.iter() {
            // if the fish is carnivorous
            if let Some(fish) = f.as_ref().is_carnivorous() {

                // looking for a meal
                // filtering out the fish that is eating
                let mut targets = self.fishes
                    .iter()
                    .filter(|&x| !same_object(f, x))
                    .collect::<Vec<_>>();
                // is my filtering done right ? I'm not sure about the collect part ...

                // picking a target at random
                let r = thread_rng().choose_mut(&mut targets);
                match r {
                    // let's eat this target
                    Some(&mut target_box) => {

                        fish.eat(target_box.as_mut());   //// issue here
                        // eat(&self, &mut fish::Fish)
                        // error : cannot borrow as mutable (on target_box in the call)
                        // but my Box is mutable thanks to choose_mut, isn't it ?

                        if !target_box.as_ref().is_alive() {
                            println!("dead fish");
                        }
                    }
                    // No target found, no meal this turn ...
                    None => println!("Cannot find a fish to eat")
                }
            }
            // else the fish is herbivorous
            else {

            }


        }