title: Why sometimes i cant set a class definition as slot in a s4 class?
body: <pre><code>r=ks.test(x=rnorm(100), "dnorm")
  class(r)
  [1] "htest"
  
  
  ## Doesnt work, "htest" is class in stats
  setClass("Jergon", representation(fit="htest"))
  [1] "Jergon"
  Warning message:
  undefined slot classes in definition of "Jergon": fit(class "htest")
  
  
  ## works "lm" 
  setClass("Jergon", representation(am="lm"))
  [1] "Jergon"
  </code></pre>