map literal in Go

Map literals are like struct literals, but the keys are required. 
If the top-level type is just a type name, you can omit it from the elements of the literal. 
```
var m = map[string]int{
	"x": 1,
	"y": 2,
}
var m = map[string]Vertex{
	"Bell Labs": {40.68433, -74.39967},
	"Google":    {37.42202, -122.08408},
}
```