function MovementSystem(grid, rules) {
  var newmover = meme(MovementSystem_prototype)
  newmover.grid  = grid;
  newmover.rules = rules || {};      // expects { canMove: fn }
  newmover.turn  = 'white';
  return newmover
}

var MovementSystem_prototype = {
  tryMove: function (piece, to) {
  if (piece.colour != this.turn) return false;

  // normalise 'to' into our hybrid coord
  var dest = [!is_null(to.x) ? to.x : to[0],
              !is_null(to.y) ? to.y : to[1]];

  if (!this.grid.inBounds(dest)) return false;
  if (!this.rules.canMove(piece, piece.coord, dest, this.grid)) return false;

  var victims = this.grid.at(dest);
  if (length(victims) && victims[0].colour == piece.colour) return false;
  if (length(victims)) victims[0].captured = true;

  this.grid.remove(piece, piece.coord);
  this.grid.add   (piece, dest);

  // grid.add() re-creates coord; re-add .x/.y fields:
  piece.coord.x = dest.x;
  piece.coord.y = dest.y;

  this.turn = (this.turn == 'white') ? 'black' : 'white';
  return true;
  }
}

return MovementSystem