Struggling with `Schema.Set` Usage in Effect TypeScript

import { Effect, ParseResult, Schema as S } from 'effect'
type A1 = {
  a?: Set<string>
}
type A2 = {
  a?: Set<string>
}

const ABSchema: S.Schema<A1, A2> = S.Struct({
  a: S.optional(S.mutable(S.Set(S.String))),
})