first 0 X nil first s X cons Y Z cons Y n__first X activate Z from X cons X n__from s X first X1 X2 n__first X1 X2 from X n__from X activate n__first X1 X2 first X1 X2 activate n__from X from X activate X X 2.2 activate n__first X1 X2 first X1 X2 activate n__from X from X first s X cons Y Z activate Z true first s X cons Y Z activate Z activate n__first X1 X2 first X1 X2 true s 1 1 n__first 2 1 2 1 activate 1 0 n__from 1 0 activate 1 1 0 0 0 0 from 1 0 first 2 1 2 0 nil 0 0 first 2 0 from 1 0 cons 2 2 28958 activate n__from X from X false NaTT certifiable-1.6