In 'A tour of Go' there is such phrase
If the top-level type is just a type name, you can omit it from the elements of the literal.
I'm new in Go so I'm curious when can not it be omitted?
var m = map[string]Vertex{
"Bell Labs": {40.68433, -74.39967}, //top-level type is omitted
"Google": {37.42202, -122.08408},
}