code: func builtinFunc(name string) func(js JSWriter, args []ast.Node) {
  	var funcStart = "soy.$$" + name + "("
  	return func(js JSWriter, args []ast.Node) {
  		js.Write(funcStart)
  		for i, arg := range args {
  			if i != 0 {
  				js.Write(",")
  			}
  			js.Write(arg)
  		}
  		js.Write(")")
  	}
  }
docs: // builtinFunc returns a function that writes a call to a soy.$$ builtin func.