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Literature.
[1] J. Komara. Declarative Programming. Downloadable lecture notes available through the web page of the course.
[2] J. Kľuka. Lecture Notes from Introduction to Declarative Programming.
[3] J. Komara and P. J. Voda. Metamathematics of Computer Programming. 2001.
Counting the labels in binary trees. The function counts the number of nodes of the binary tree labelled by the number .