[第1回] string.operator+(string, string)の説明を追加する

[kztk-m]

後半の式の説明のところで説明する．.NETのStringオブジェクト由来ではなく，C#の言語機能の一部という理解なので説明に気をつける．

https://learn.microsoft.com/en-us/dotnet/api/system.string.concat?view=net-8.0 曰く：

You can also use your language's string concatenation operator, such as + in C# and F#, or & and + in Visual Basic, to concatenate strings. Both compilers translate the concatenation operator into a call to one of the overloads of String.Concat.