How to use the @jsii/spec.isUnionTypeReference function in @jsii/spec

case spec.PrimitiveType.Json: return [{ serializationClass: SerializationClass.Json, typeRef }];
      case spec.PrimitiveType.Boolean:
      case spec.PrimitiveType.Number:
      case spec.PrimitiveType.String:
        return [{ serializationClass: SerializationClass.Scalar, typeRef }];
    }

    throw new Error('Unknown primitive type');
  }
  if (spec.isCollectionTypeReference(typeRef.type)) {
    return [{
      serializationClass: typeRef.type.collection.kind === spec.CollectionKind.Array ? SerializationClass.Array : SerializationClass.Map,
      typeRef
    }];
  }
  if (spec.isUnionTypeReference(typeRef.type)) {
    const compoundTypes = flatMap(typeRef.type.union.types, t => serializationType({ type: t }, lookup));
    // Propagate the top-level 'optional' field to each individual subtype
    for (const t of compoundTypes) {
      if (t.typeRef !== 'void') {
        t.typeRef.optional = typeRef.optional;
      }
    }
    return compoundTypes.sort((l, r) => compareSerializationClasses(l.serializationClass, r.serializationClass));
  }

  // The next part of the conversion is lookup-dependent
  const type = lookup(typeRef.type.fqn);

  if (spec.isEnumType(type)) {
    return [{ serializationClass: SerializationClass.Enum, typeRef }];
  }

public get unionOfTypes(): TypeReference[] | undefined {
    if (!jsii.isUnionTypeReference(this.spec)) {
      return undefined;
    }

    return this.spec.union.types.map(t => new TypeReference(this.system, t));
  }
}

function _collectTypeReferences(type: spec.TypeReference): void {
    if (spec.isNamedTypeReference(type)) {
      typeReferences.push(type);
    } else if (spec.isCollectionTypeReference(type)) {
      _collectTypeReferences(type.collection.elementtype);
    } else if (spec.isUnionTypeReference(type)) {
      type.union.types.forEach(_collectTypeReferences);
    }
  }
}