How to understand なんだ


In the following sentence:

図書館へ行くつもりなんだ

What is なんだ from the grammar point of view?




I guess I was right, so I'll offer it as an answer.

[According to
Daijirin](https://kotobank.jp/word/%E3%81%AA%E3%82%93%E3%81%A0-590581#E5.A4.A7.E8.BE.9E.E6.9E.97.20.E7.AC.AC.E4.B8.89.E7.89.88)
(look at the bottom of the link) it is a shortened form of the 連語 `なのだ`.




I guess I was right, so I'll offer it as an answer.

[According to
Daijirin](https://kotobank.jp/word/%E3%81%AA%E3%82%93%E3%81%A0-590581#E5.A4.A7.E8.BE.9E.E6.9E.97.20.E7.AC.AC.E4.B8.89.E7.89.88)
(look at the bottom of the link) it is a shortened form of the 連語 `なのだ`.

