code: func (f *Field) RotatePieceLeft(piece *Piece, x, y int, angle Angle) Angle {
  	if piece.collides(f, x, y, angle.RotateLeft()) {
  		return angle
  	}
  	return angle.RotateLeft()
  }
docs: // RotatePieceLeft tries to rotate the piece to the left
  // and returns the piece's next angle.