title: Append arguments to an s4 Class
body: <p>Suppose, one forgets to set <code>validity</code> in an s4 class definition, how could one "append" this argument without rewriting the entire class.</p>
  
  <p>Illustration:</p>
  
  <pre><code>setClass("test",slots = c(Taken="numeric",Data="data.frame"))
  x&lt;-new("test",Taken=123,Data=data.frame(GPA=0.02,Score=0.01))
  </code></pre>
  
  <p>Now, suppose I would like to check validity:</p>
  
  <pre><code>validity.test&lt;-function(object){
    if(!all(sapply(object@Data,is.numeric))){
      print("Data must be all numeric")
  
    } else print(TRUE)
  
  }
  </code></pre>
  
  <p>I could just call <code>validity.test(x)</code>. </p>
  
  <p>However, how do I set it to <code>test</code> without rewriting <code>test</code>?! 
  We are working on the assumption that someone is new(like me) to s4 and is therefore likely to forget doing this. If the script is so many lines, this can easily get tiresome.
  Thanks in advance!</p>