dbl 0 0 dbl s X s s dbl X dbls nil nil dbls cons X Y cons dbl X dbls Y sel 0 cons X Y X sel s X cons Y Z sel X Z indx nil X nil indx cons X Y Z cons sel X Z indx Y Z from X cons X from s X 2.2 sel s X cons Y Z sel X Z dbls cons X Y dbl X dbls cons X Y dbls Y indx cons X Y Z indx Y Z from X from s X dbl s X dbl X indx cons X Y Z sel X Z true from X from s X true Failed! NaTT certifiable-1.6