P.S. With no bracket at the save line...

squarefree := proc()
    return true;
end proc;

save squarefree, "C:\\Users\\Desktop\\project\\lib\\m\\Auxiliary.m");

I will try this tomorrow!

The Iterator package is perfect for all my work. thank you! That solves everything very quickly!

Thank you so much!

with(combinat):
choose({1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20}, 10):

is no option, that takes over 50 % more time (3,8 sec)  than my own algorithm (1,8 sec). There must be a faster way.

Thank you all for your great help!