declare
class ListClass
   meth isNil(_) raise undefinedMethod end end
   meth append(_ _) raise undefinedMethod end end
   meth display raise undefinedMethod end end
end

class NilClass from ListClass
   meth init skip end
   meth isNil(B) B=true end
   meth append(T U) U=T end
   meth display {Browse nil} end
end

class ConsClass from ListClass
   attr head tail
   meth init(H T) head:=H tail:=T end
   meth isNil(B) B=false end
   meth append(T U)
      U2={@tail append(T $)}
   in
      U={New ConsClass init(@head U2)}
   end
   meth display {Browse @head} {@tail display} end
end