A membership written, for the compilers, as the statement it makes about its element; the set is fixed
when the expression is compiled, so only the element is computed. In a union, an intersection or a
difference, Kleene's connectives of the memberships in its sides, as evaluation decides them; in an
interval, the element real and between the ends; in a finite set, equal to one of its members; and in a
conditional set, the set's predicate of it. A membership in a set of numbers is left, for each compiler to
compile itself, and one in any other set, or with any other set inside, is left as written, which neither
compiles.
https://github.com/asc-community/AngouriMath/issues/2195