code: func (f *Field) MovePieceToLeft(piece *Piece, x, y int, angle Angle) int {
  	if piece.collides(f, x-1, y, angle) {
  		return x
  	}
  	return x - 1
  }
docs: // MovePieceToLeft tries to move the piece to the left
  // and returns the piece's next x position.