if I have a checkboxGroup defined as

```
 checkboxGroupInput("checkGroup", label = h2("Data Sets"), 
                     choices = list("Accounts Created" = 1, "Comments" = 2,
                                    "Submissions" = 3, "Votes" = 4),
                     selected = NULL)

```

Then on the server side, how do I access the selection status of the four boxes? Is it `input$checkGroup[1]`, `input$checkGroup[2]`, etc with 0/1 values? Or is `input$checkGroup` variable length containing only the values selected? Very confused as to how to determine if, for example, the first and third boxes are checked...