Function union

src/utils/set.ts:44–53

(a: Set<A>, b: Set<A>)

 * Note: this code is hot, so is optimized for speed.
 */
export function union<A>(a: Set<A>, b: Set<A>): Set<A> {
  const result = new Set<A>()
  for (const item of a) {
    result.add(item)
  }
  for (const item of b) {
    result.add(item)
  }
  return result
}