Many thanks Raymond for these insights and thorough explanations, I appreciate it very much. Your conclusions are of course most reasonable. For me, I cannot say that it is a real issue in practice. I have teaching notes in which I illustrate with plots the differences in complexity when working with sets versus lists. One such plot showed that the cost of removing an element from a set does not depend on what the element is (which is why I reused the same element again and again) and is orders of magnitude less than for removing an element from the middle of a list. I just found out that something had changed with Python3.9 when I last run the cell of the jupyter notebook in which that plot was produced...