Apollo Federation resolving requests for entities


I am implementing a federated service and have added the federation schema specs found here. Apollo Federation subgraph specification - Federation - Apollo GraphQL Docs

scalar _Any
scalar _FieldSet

# a union of all types that use the @key directive
union _Entity

type _Service {
  sdl: String

extend type Query {
  _entities(representations: [_Any!]!): [_Entity]!
  _service: _Service!

directive @external on FIELD_DEFINITION
directive @requires(fields: _FieldSet!) on FIELD_DEFINITION
directive @provides(fields: _FieldSet!) on FIELD_DEFINITION
directive @key(fields: _FieldSet!) repeatable on OBJECT | INTERFACE

# this is an optional directive discussed below
directive @extends on OBJECT | INTERFACE

Now I am having trouble implementing (or resolving) the _entities field on the query root. In other words, I am unsure how best to resolve the _entities Query method,
_entities(representations: [_Any!]!): [_Entity]!.

I have not been able to find examples of how the _entities Query can be implemented to successfully query a federated service. Any advice would be appreciated. Thanks in advance.

I am using @graphql-tools to add resolvers to schema and need to define _entities Query:

const resolvers = {
  Query: {
    libraries() {
      return libraries;
    _service() {
      return service;
    // _entities() {}

Thanks in advance.