Choosing a value or pointer receiver
There are two reasons to use a pointer receiver.
The first is so that the method can modify the value that its receiver points to.
The second is to avoid copying the value on each method call. This can be more efficient if the receiver is a large struct, for example.
In this example, both Scale and Abs are methods with receiver type *Vertex, even though the Abs method needn't modify its receiver.
In general, all methods on a given type should have either value or pointer receivers, but not a mixture of both. (We'll see why over the next few pages.)
Interfaces
An interface type is defined as a set of method signatures.
A value of interface type can hold any value that implements those methods.
Note: There is an error in the example code on line 22. Vertex (the value type) doesn't implement Abser because the Abs method is defined only on *Vertex (the pointer type).