Jaehunt:
Does any have a idea for it?
Unfortunately this is a design limitation of D operator overloading. I have shown this problem in the main D newsgroup lot of time ago. I don't remember the answer, but nothing changed. You can bring the problem again in the main D newsgroup...
In the meantime you have to define UFCS functions as: s1.isSubset(s2) s2.isStrictSubset(s2) Bye, bearophile
