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 l1 ifappend l1 l2 nil l2 ifappend l1 l2 cons x l cons x append l l2 2.2 append l1 l2 ifappend l1 l2 l1 ifappend l1 l2 cons x l append l l2 true ifappend l1 l2 cons x l append l l2 append l1 l2 ifappend l1 l2 l1 true hd 1 0 append 2 1 1 is_empty 1 0 ifappend 3 0 false 0 0 true 0 0 tl 1 0 append 2 0 tl 1 0 nil 0 0 is_empty 1 0 hd 1 0 cons 2 2 2 ifappend 3 3 0 NaTT certifiable-1.6