The problem has been solved. I just had to use print(klassen)
I am not sure if I am allowed to post about R questions, but I will just go ahead:
The problem is explained here:
R is not printing my classes correctly when I try to use cat() to do it.
If I just embed the 'klassen' command in the body of my makeClasses() function, R does not print them out neither.
So, I would like to print my intervals correctly.