is_empty nil true is_empty cons x l false hd cons x l x tl cons x l l append l1 l2 ifappend l1 l2 is_empty l1 ifappend l1 l2 true l2 ifappend l1 l2 false cons hd l1 append tl l1 l2 2.2 ifappend l1 l2 false append tl l1 l2 append l1 l2 is_empty l1 ifappend l1 l2 false tl l1 ifappend l1 l2 false hd l1 append l1 l2 ifappend l1 l2 is_empty l1 true append l1 l2 ifappend l1 l2 is_empty l1 ifappend l1 l2 false append tl l1 l2 true Failed! NaTT certifiable-1.6