/// <summary>
/// Builds the player and adds it to given session
/// </summary>
/// <param name="toSession">
/// Session which player will added to.
/// </param>
/// <returns>
/// <see cref="Player"/>
/// </returns>
/// <exception cref="InvalidOperationException"></exception>
public Player Build(ISession toSession)
{
if (_player.Stones.Count(s => s is King) == 0)
{
throw new InvalidOperationException("Player has no King stone!");
}
toSession.AddPlayer(_player);
return(_player);
}