Handling Cleanup in Stream.flatMap with `{ switch: true }`

type Validator = 'a' | 'b' | 'c';
type Result = 'loading' | 'pass' | 'fail'
// Stream<never, never, T> is abbrevated to Stream<T> for simplicity.
declare const validationProgram: (input: Stream<string>) => Stream<Record<Validator, Result>>
// Example
// input: x ------ start validate('a', x') validate('b', x) validate('c', x) in parallel
// output: { a: 'loading', b: 'loading', c: 'loading' }
// ... 
// output: { a: 'loading', b: 'pass',    c: 'loading' }
// input: y ----- cancel validate('a', x) and validate('c', x) and start over with y
// output: { a: 'loading', b: 'loading', c: 'loading' }
// ...
// output: { a: 'loading', b: 'pass',    c: 'loading' }
// ...
// output: { a: 'fail'   , b: 'pass',    c: 'loading' }
// ...
// output: { a: 'fail'   , b: 'pass',    c: 'pass'    }

function validationProgram(
  input: Stream<string>
) : Stream<Record<Validator, Result>> {
  return pipe(
    inputStream,
    Stream.debounce(300),
    Stream.flatMap((input) => {
      const resultStream = /* how to do this with proper cleanup */
      return pipe(
        Stream.sync(() => ({
          a: 'loading', b: 'loading', c: 'loading'
        })),
        Stream.concat(resultStream)
      )
    }, { switch: true })
  )
}