来自gin-gonic文档的语法问题

I have been developing in Node/Python for years, and over the holiday I have been trying to expand my knowledge of Go. I have a pet project that I've been working on to learn it.

As I have been reading the gin-gonic documentation, I found syntax that I can't wrap my head around.

func main() {
    router := gin.Default()

    // Simple group: v1
    v1 := router.Group("/v1")
    {
        v1.POST("/login", loginEndpoint)
        v1.POST("/submit", submitEndpoint)
        v1.POST("/read", readEndpoint)
    }

    // Simple group: v2
    v2 := router.Group("/v2")
    {
        v2.POST("/login", loginEndpoint)
        v2.POST("/submit", submitEndpoint)
        v2.POST("/read", readEndpoint)
    }

    router.Run(":8080")
}

Basically, it looks like the router.Group() method takes in a struct, but I am not quite sure what the mechanics are here... It's not wrapped in parentheses. How is this object being passed in/handled by the Group method?

The Go Programming Language Specification

Blocks

A block is a possibly empty sequence of declarations and statements within matching brace brackets.

Declarations and scope

Go is lexically scoped using blocks.

For example, the v1.POST method call statements form a block:

// Simple group: v1
v1 := router.Group("/v1")
{
    v1.POST("/login", loginEndpoint)
    v1.POST("/submit", submitEndpoint)
    v1.POST("/read", readEndpoint)
}

This is an unusual use of blocks.