■

$ scala
...
scala> trait X { def run }
defined trait X

scala> var x: X = null
x: X = null

scala> class Person extends X {
     |   def run = println("two legs")
     | }
defined class Person

scala> class Dog extends X {
     |   def run = println("four legs")
     | }
defined class Dog

scala> class Car extends X {
     |   def run = println("wheel tracks")
     | }
defined class Car

scala> class Computer extends X {
     |   def run = println("move about")
     | }
defined class Computer