title: checkboxGroupInput - set minimum and maximum number of selections - ticks
body: <p>Here is example code with check-box group input:</p>
  
  <pre><code>library(shiny)
  
  server &lt;- function(input, output) {
    output$Selected &lt;- renderText({
      paste(input$SelecetedVars,collapse=",")
    })
  }
  
  ui &lt;- fluidPage(
    sidebarLayout(
      sidebarPanel(
        checkboxGroupInput("SelecetedVars", "MyList:",
                           paste0("a",1:5), selected = "a1")
      ),
      mainPanel(textOutput("Selected"))
    )
  )
  
  shinyApp(ui = ui, server = server)
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/6DcZP.png" alt="enter image description here"></p>
  
  <p>As you you can see from image above we can select as many as we want, in this case 4 out of 5.</p>
  
  <p>How can I set minimum and maximum number of ticks? I need minimum of 1 option checked and maximum of 3 options checked. i.e.: prevent <em>unticking</em> the last tick, and prevent ticking when 3 options were already <em>ticked</em>. </p>