const link = returnOne.concat(mock).concat(op => {
   expect(op.getContext()).toEqual({
     add: 3,
     substract: 1,
   });
   return Observable.of({ data: op.getContext().add });
 });

 const link = new ApolloLink(op => {
   expect(operation['operationName']).toBeUndefined();
   expect(operation['variables']).toBeUndefined();
   expect(operation['context']).toBeUndefined();
   expect(operation['extensions']).toBeUndefined();
   expect(op['operationName']).toBeDefined();
   expect(op['variables']).toBeDefined();
   expect(op['context']).toBeUndefined();
   expect(op['extensions']).toBeDefined();
   expect(op.toKey()).toBeDefined();
   return Observable.of();
 });

 operation => Observable.of({ data: { count: 2 } }),

 const link = new MockLink(operation => Observable.of());