How to understand なんだ [duplicate]

Question

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?

Answer 1

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

According to Daijirin (look at the bottom of the link) it is a shortened form of the 連語 なのだ.