I believe it was things like "+" and "-" for set union and subtraction, etc.
That, or &, |, and ^, by analogy with bitwise operators. It definitely came up during the SetAlgebra discussions.
Another thread I guess I didn’t follow closely enough. I think I agree with avoiding using unconventional operators for operations which already have conventional operators associated with them. Maybe someday it will be easy enough to type unicode operators that it might be reasonable to think about using them.
Yes, there is precedent in other languages:
Ceylon uses "&" for type intersection (any<>) and "|" for type union (one<>), see http://ceylon-lang.org/documentation/1.2/tour/types/.
TypeScript does the same (see https://www.typescriptlang.org/docs/handbook/advanced-types.html).