public String print(String tab) {
   return tab
       + "type = "
       + type
       + Out.NL
       + tab
       + "child 1 :"
       + Out.NL
       + //$NON-NLS-1$ //$NON-NLS-2$
       r1.print(tab + "  ")
       + Out.NL
       + tab
       + "child 2 :"
       + Out.NL
       + //$NON-NLS-1$ //$NON-NLS-2$
       r2.print(tab + "  "); // $NON-NLS-1$
 }