diff --git a/js/iterator.js b/js/iterator.js
new file mode 100644
index 0000000..db0c086
--- /dev/null
+++ b/js/iterator.js
@@ -0,0 +1,136 @@
+'use strict';
+
+export const IteratorMixin = {
+   map: function* map(f) {
+      let y;
+      while (!(y = this.next()).done) {
+         yield f(y.value);
+      }
+      return y.value;
+   },
+
+   filter: function* filter(f) {
+      let y;
+      while (!(y = this.next()).done) {
+         if (f(y.value)) {
+            yield y.value;
+         }
+      }
+      return y.value;
+   },
+
+   take: function* take(limit) {
+      let remaining = Number(limit) & -1;
+      let y;
+      while (remaining > 0 && !(y = this.next()).done) {
+         yield y.value;
+         remaining -= 1;
+      }
+   },
+
+   drop: function* drop(limit) {
+      let remaining = Number(limit) & -1;
+      let y;
+      while (!(y = this.next()).done) {
+         if (remaining <= 0) {
+            yield y.value;
+         } else {
+            remaining -= 1;
+         }
+      }
+   },
+
+   asIndexedPairs: function* asIndexedPairs() {
+      let index = 0;
+      let y;
+      while (!(y = this.next()).done) {
+         yield [index++, x];
+      }
+      return y.value;
+   },
+
+   flatMap: function* flatMap(f) {
+      let y;
+      while (!(y = this.next()).done) {
+         yield* f(y.value);
+      }
+      return y.value;
+   },
+
+   reduce: function reduce(f, state) {
+      if (typeof state === 'undefined') {
+         const first = this.next();
+         if (first.done) {
+            throw new TypeError('reduce: empty iterator');
+         }
+         state = first.value;
+      }
+      let y;
+      while (!(y = this.next()).done) {
+         state = f(state, y.value);
+      }
+      return state;
+   },
+
+   toArray: function toArray() {
+      return [...this];
+   },
+
+   forEach: function* forEach(f) {
+      let y;
+      while (!(y = this.next()).done) {
+         f(y.value);
+      }
+   },
+
+   some: function some(f) {
+      /* extension: if f is undefined, assume identity function */
+      let iter = (typeof f === 'undefined') ? this : IteratorMixin.map.call(this, f);
+      let y;
+      while (!(y = iter.next()).done) {
+         if (y.value) {
+            return true;
+         }
+      }
+      return false;
+   },
+
+   every: function every(f) {
+      /* extension: if f is undefined, assume identity function */
+      let iter = (typeof f === 'undefined') ? this : IteratorMixin.map.call(this, f);
+      let y;
+      while (!(y = iter.next()).done) {
+         if (!y.value) {
+            return false;
+         }
+      }
+      return true;
+   },
+
+   find: function find(f) {
+      if (typeof f === undefined) {
+         /* extension */
+         f = function identity(x) { return x; };
+      }
+      let y;
+      while (!(y = this.next()).done) {
+         if (f(y.value)) {
+            return y.value;
+         }
+      }
+   },
+
+   /* extension */
+   includes: function includes(x) {
+      return IteratorMixin.some.call(this, function equalsX(y) { return y === x; });
+   },
+};
+
+export const Iterator = {};
+for (const fn in IteratorMixin) {
+   Iterator[fn] = function() {
+      return IteratorMixin[fn].call(...arguments);
+   };
+}
+
+export default Iterator;
diff --git a/js/pacosako.js b/js/pacosako.js
index 5e563c7..633d1f9 100644
--- a/js/pacosako.js
+++ b/js/pacosako.js
@@ -1,15 +1,6 @@
 'use strict';
-(function(factory) {
-  if (typeof define !== 'undefined' && define.amd) {
-    define([], factory);
-  } else if (typeof define !== 'undefined' && define.petal) {
-    define(['paco-sako'], [], factory);
-  } else if (typeof module !== 'undefined' && module.exports) {
-    module.exports = factory();
-  } else {
-    window.PacoSako = factory();
-  }
-})(function(){
+
+import {Iterator} from './iterator.js';
 
 /* Game states */
 const PLAYING = 'playing';
@@ -136,6 +127,10 @@ class Board {
       }
    }
 
+   toString() {
+      return new Buffer(this._board).toString('hex')
+   }
+
    get phantom() {
       let phantom = this._phantom;
       if (phantom) {
@@ -236,7 +231,7 @@ class Board {
       return ((theirs === EMPTY) ? (ours === EMPTY) : (canCapture ? true : false));
    }
 
-   scanPath(accum, side, from, canCapture, columnsRight, rowsUp, remainder) {
+   *scanPath(side, from, canCapture, columnsRight, rowsUp, remainder) {
       while (true) {
          let there = offsetSquare(from, columnsRight, rowsUp);
 
@@ -244,7 +239,7 @@ class Board {
             return;
          }
 
-         accum.push(there);
+         yield there;
 
          if (remainder < 1 || this.getPiece(otherSide(side), there) !== EMPTY) {
             return;
@@ -255,26 +250,41 @@ class Board {
       }
    }
 
-   findPieces(side, type, alongside) {
+   *findPieces(side, type, alongside) {
       const other = otherSide(side);
-      let pieces = [];
 
-      if (this._phantom && this._phantom.side === side && this._phantom.type === type) {
-         pieces.push(this._phantom.from);
+      function testFunction(getPiece, match) {
+         if (match === undefined) {
+            return function(here) { return true; }
+         } else if (match === false) {
+            return function(here) { return getPiece(here) === EMPTY; };
+         } else if (match === true) {
+            return function(here) { return getPiece(here) !== EMPTY; };
+         } else {
+            return function(here) { return getPiece(here) === match; };
+         }
+      }
+
+      const testSide = testFunction(this.getPiece.bind(this, side), type);
+      const testOther = testFunction(this.getPiece.bind(this, other), alongside);
+      const test = function(here) { return testSide(here) && testOther(here); };
+
+      if (this._phantom && this._phantom.side === side) {
+         const getPhantom = (here) => this._phantom.type;
+         const testPhantom = testFunction(getPhantom, type);
+         if (testPhantom(PHANTOM)) {
+            yield this._phantom.from;
+         }
       }
 
       for (const row of ROWS) {
          for (const column of COLUMNS) {
             const here = column + row;
-            if (this.getPiece(side, here) === type) {
-               if (!alongside || this.getPiece(other, here) === alongside) {
-                  pieces.push(here);
-               }
+            if (test(here)) {
+               yield here;
             }
          }
       }
-
-      return pieces;
    }
 }
 
@@ -347,12 +357,12 @@ class Game {
       }
    }
 
-   legalMoves(side, from, canCapture) {
+   *legalMoves(side, from, canCapture) {
       const board = this._board;
       const type = board.getPiece(side, from);
 
       if (this._status !== PLAYING || type === EMPTY) {
-         return [];
+         return;
       }
 
       if (canCapture === undefined) {
@@ -364,11 +374,9 @@ class Game {
          from = board.phantom.from;
       }
 
-      let legals = [];
-
       if (type === KING) {
          for (const dir of ANY_DIR) {
-            board.scanPath(legals, side, from, false, dir[0], dir[1], 0);
+            yield* board.scanPath(side, from, false, dir[0], dir[1], 0);
          }
 
          /* check for castling conditions */
@@ -378,34 +386,34 @@ class Game {
             if (this._castling[side].king &&
                   board.isEmpty('f' + row) &&
                   board.isEmpty('g' + row)) {
-               legals.push('g' + row);
+               yield 'g' + row;
             }
 
             if (this._castling[side].queen &&
                   board.isEmpty('d' + row) &&
                   board.isEmpty('c' + row) &&
                   board.isEmpty('b' + row)) {
-               legals.push('c' + row);
+               yield 'c' + row;
             }
          }
       } else if (type === QUEEN) {
          for (const dir of ANY_DIR) {
-            board.scanPath(legals, side, from, canCapture, dir[0], dir[1], 8);
+            yield* board.scanPath(side, from, canCapture, dir[0], dir[1], 8);
          }
       } else if (type === BISHOP) {
          for (const dir of DIAG) {
-            board.scanPath(legals, side, from, canCapture, dir[0], dir[1], 8);
+            yield* board.scanPath(side, from, canCapture, dir[0], dir[1], 8);
          }
       } else if (type === KNIGHT) {
          for (const dir of KNIGHT_DIR) {
             const there = offsetSquare(from, dir[0], dir[1]);
             if (there && board.validDestination(side, there, canCapture)) {
-               legals.push(there);
+               yield there;
             }
          }
       } else if (type === ROOK) {
          for (const dir of ORTHO) {
-            board.scanPath(legals, side, from, canCapture, dir[0], dir[1], 8);
+            yield* board.scanPath(side, from, canCapture, dir[0], dir[1], 8);
          }
       } else if (type === PAWN) {
          const dark = side === DARK;
@@ -415,10 +423,10 @@ class Game {
          const diagR = offsetSquare(from,  1, dark ? -1 : 1);
 
          if (forward && board.validDestination(side, forward, false)) {
-            legals.push(forward);
+            yield forward;
             if (dark ? (from[1] >= '7') : (from[1] <= '2')) {
                if (forward2 && board.validDestination(side, forward2, false)) {
-                  legals.push(forward2);
+                  yield forward2;
                }
             }
          }
@@ -427,7 +435,7 @@ class Game {
             for (const there of [diagL, diagR]) {
                if (there) {
                   if (board.getPiece(otherSide(side), there) !== EMPTY) {
-                     legals.push(there);
+                     yield there;
                   } else if (forward2) {
                      let lastMove = null;
                      for (let i = this._moves.length; i > 0; --i) {
@@ -442,15 +450,29 @@ class Game {
                            lastMove.from === there[0] + forward2[1] &&
                            lastMove.to === there[0] + from[1]) {
                         /* en passant */
-                        legals.push(there);
+                        yield there;
                      }
                   }
                }
             }
          }
       }
+   }
 
-      return legals;
+   isLegalMove(side, from, to, canCapture) {
+      const legals = this.legalMoves(side, from, canCapture);
+      return Iterator.some(legals, (where) => where === to);
+   }
+
+   isInCheck(_side) {
+      const side = _side || this._player;
+      const kings = [...this._board.findPieces(side, KING)];
+
+      if (kings.length !== 1) {
+         throw { message: "there should be exactly one king" };
+      }
+
+      const king = kings[0];
    }
 
    move(from, to, meta) {
@@ -472,8 +494,7 @@ class Game {
 
       const fromSquare = (from === PHANTOM) ? board.phantom.from : from;
 
-      const legals = this.legalMoves(side, from, alongside === EMPTY);
-      if (!legals.includes(to)) {
+      if (!this.isLegalMove(side, from, to, alongside === EMPTY)) {
          throw { message: "illegal move", side: side, from: fromSquare, to: to };
       }
 
@@ -724,7 +745,7 @@ class Game {
                let sameRank = 0;  /* row / number */
 
                for (const where of sameKind) {
-                  if (replay.legalMoves(move.side, where, true).includes(move.to)) {
+                  if (replay.isLegalMove(move.side, where, move.to, true)) {
                      legalFrom.push(where);
 
                      if (where[0] === move.from[0]) {
@@ -783,34 +804,24 @@ class Game {
    }
 }
 
-return {
-   Board: Board,
-   Game: Game,
+export default {
+   /* Classes */
+   Board, Game,
 
-   PLAYING: PLAYING,
-   ENDED: ENDED,
+   /* Game States */
+   PLAYING, ENDED,
 
-   LIGHT: LIGHT,
-   DARK: DARK,
+   /* Sides */
+   LIGHT, DARK,
 
-   KING: KING,
-   QUEEN: QUEEN,
-   ROOK: ROOK,
-   KNIGHT: KNIGHT,
-   BISHOP: BISHOP,
-   PAWN: PAWN,
-   EMPTY: EMPTY,
+   /* Pieces */
+   KING, QUEEN, ROOK, KNIGHT, BISHOP, PAWN, EMPTY,
 
-   ROWS: ROWS,
-   COLUMNS: COLUMNS,
-   PHANTOM: PHANTOM,
+   /* Coordinates */
+   ROWS, COLUMNS, PHANTOM,
 
-   Util: {
-      otherSide: otherSide,
-      offsetSquare: offsetSquare,
-   },
+   /* Miscellaneous */
+   Util: { otherSide, offsetSquare },
 };
 
-});
-
 /* vim:set expandtab sw=3 ts=8: */