package main import (     "fmt"     "math" ) type Abser interface {     Abs() float64 } func main() {     var a Abser     f := MyFloat(-math.Sqrt2)     v := Vertex{3, 4}     a = f  // a MyFloat implements Abser     a = &v // a *Vertex implements Abser     a = v  // a Vertex, does NOT            // implement Abser     fmt.Println(a.Abs()) } type MyFloat float64 func (f MyFloat) Abs() float64 {     if f < 0 {         return float64(-f)     }     return float64(f) } type Vertex struct {     X, Y float64 } func (v *Vertex) Abs() float64 {     return math.Sqrt(v.X*v.X + v.Y*v.Y) }