When I started the Ranged Sets library "infinite" sets (i.e. sets based on infinite lists of ranges) looked easy. The set primitives of union and intersection are simple merge algorithms, so they would work fine on infinite lists of ranges. Set difference is implemented as a combination of intersection and negation, and negation is just a simple rearrangement of the range end points, so that works too. However there are cases where this reasoning doesn't work. Suppose I have two ranged sets based on Range (BoundaryBelow n) (BoundaryBelow (n+1)) Set X contains these ranges for all even values of n>0 Set Y contains these ranges for all odd values of n>0 The union of these sets should be the single range Range (BoundaryBelow 1) BoundaryAboveAll In practice however the naive merge algorithm will never terminate because it has to merge an infinite number of ranges into the single range above. A similar problem occurs with intersection. The intersection of these sets should be empty, but again the merge algorithm will iterate forever looking for the first range in the set. As a workaround in 0.0.2 I've put a counter in. The intersection routine adds a null empty range after 10 empty iterations, and the union routine puts a break between touching ranges after 10 full iterations. Hence set membership tests are guaranteed to terminate in the presence of infinite lists. However the same cannot be said of all the functions. Set equality in particular has to evaluate the entire set in order to return True (although it does terminate at the first counter-example). The question of whether a particular expression is guaranteed to terminate is not trivial if it contains even one infinite list. So my question is this: are infinite sets worth bothering about? Should I continue to put in fixes to ensure termination in at least some cases? Or should I just declare that infinite lists of ranges are unsupported? Thanks, Paul.