title: Combining child class lists to parent class list
body: <p>I have a parent class X and two lists of child classes X1 and X2 (X1 and X2 derive from X). Is there an easy way to combine the lists of X1 and X2 into a list of Xs?<br>
  I could loop over the lists of X1 and X2 and add then to the list of X but this seems a bit cumbersome.</p>