docs: function into(target, collection, xform) {
    switch (true) {
      case isArray(target):
        return transduce(collection, xform, arrayReducer, target);
      case isObject(target):
        return transduce(collection, xform, objectReducer, target);
      case isString(target):
        return transduce(collection, xform, stringReducer, target);
      case isImplemented(target, 'step'):
        return transduce(collection, xform, target, target);
      default:
        throw Error(`Cannot reduce collection into ${target}: ${collection}`);
    }
  }