Fix mk_starts and second_last in knights benchmarks.

mk_starts was not yielding enough values. It's originally a translation of a double list comprehension in Haskell, which cannot simply be translated to a map2. The latter combine elements two by two, but the former works through all possible permutations.

examples/benchmarks/lib/benchmarks/knights.ak

@@ -44,14 +44,20 @@ fn root(size: Int) -> Queue<(Int, ChessSet)> {
   queue.append_all_front(queue.new(), mk_starts(size))
 }
 
-fn mk_starts(sze: Int) -> List<(Int, ChessSet)> {
-  let x_list = interval(1, sze)
-  let y_list = interval(1, sze)
+fn mk_starts(size: Int) -> List<(Int, ChessSet)> {
+  let it = interval(1, size)
+
+  let l =
+    list.flat_map(
+      it,
+      fn(x) { list.map(it, fn(y) { start_tour((x, y), size) }) },
+    )
 
-  let l = x_list |> list.map2(y_list, fn(a, b) { start_tour((a, b), sze) })
   let length = list.length(l)
 
-  list.repeat(1 - length, length) |> list.zip(l)
+  expect length == size * size
+
+  list.zip(list.repeat(1 - length, length), l)
 }
 
 fn interval(a: Int, b: Int) -> List<Int> {

examples/benchmarks/lib/benchmarks/knights/chess_set.ak

@@ -33,32 +33,21 @@ pub fn last_piece(board: ChessSet) -> Tile {
 pub fn delete_first(board: ChessSet) -> ChessSet {
   let ChessSet { move_number, visited, .. } = board
 
-  expect Some(deleted_first) = list.init(visited)
+  expect Some(new_visited) = list.init(visited)
 
   ChessSet {
     ..board,
     move_number: move_number - 1,
     start: second_last(visited),
-    visited: deleted_first,
+    visited: new_visited,
   }
 }
 
 fn second_last(visited: List<a>) -> Option<a> {
-  when visited is {
-    [] -> None
-    [_, ..rest] -> {
-      let value = second_last(rest)
-
-      if value == None {
-        if builtin.null_list(rest) {
-          None
-        } else {
-          Some(builtin.head_list(visited))
-        }
-      } else {
-        value
-      }
-    }
+  when list.reverse(visited) is {
+    [] -> fail
+    [_] -> None
+    [_, a, ..] -> Some(a)
   }
 }