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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.
Deletion of the first label. Define the function such that
Hint. Note that the application yields root element for non-empty trees.