The typology constrains the scope of "membership" in a specific way. Also, the construction of the typology depends upon the way this constraint works.

The rule "A set cannot be a member of itself" sounds simple but isn't. When it's modified to avoid 'overkill' it rapidly becomes complex - only becoming intelligible again when it approaches very closely to "A set cannot be a member of itself if this would result in contradiction" - which, of course, is exactly what we're trying to avoid by other means.

This pattern is beginning to sound familiar ... Which means, I suppose, that I may have made it up.