Operations over sets

setops.js

const union = (...sets) => new Set(sets.flatMap(s => [...s]))
const intersection = (a, b, ...rest) => (rest.length ? intersection : (s => new Set(s)))([...a].reduce((acc, e) => b.has(e) ? [...acc, e] : acc, []), ...rest)
const difference = (...sets) => [...intersection(...sets)].reduce((acc, e) => !acc.delete(e) || acc, union(...sets))
const leftDifference = (left, ...rights) => difference(left, intersection(left, union(...rights)))
const rightDifference = (...sets) => leftDifference(...sets.reverse())