There are a number of problems with this argument, but the biggest one is that most such calls will not actually be tail-recursive. The very specific `adjoin` e

There are a number of problems with this argument, but the biggest one is that most such calls will not actually be tail-recursive. The very specific `adjoin` example that has been provided only works with `contains` not being in a tail position because `or` is assumed to use short-circuit evaluation, and because the author is apparently willing to live with a set membership test that has linear time complexity. We can see that the `contains` method for `UnionObject` is not tail-recursive, for example, no matter how you cut it. The primes example effectively constructs a linked list with 78,498 elements; any unsuccessful search will iterate over all those elements, and successful searches will (depending on how the search arguments are distributed) also easily traverse tens of thousands of elements. Handwaving performance issues away with "we know lots of ways this code can be optimized, but let us please focus on the behavior of the constructed set" is wrong, simply because efficient and actually usable implementations may exhibit totally different requirements w.r.t. tail recursion and what should be in an interface.