Understanding the `_` parameter in Effect.gen (underscore placeholder)

const getPokemon = (id: number) =>
  Effect.gen(function* () {
    const res = yield* Effect.tryPromise({
      try: () => fetch(`https://pokeapi.co/api/v2/pokemon/${id}`).then((res) => res.json()),
      catch: () => new Error('error fetching pokemon'),
    });

    return yield* parsePokemon(res);
  });