+act #15804 blackbox verification for head subscriber
=str: #16285: Expand should not drop elements on completion
Merge pull request #16387 from drewhk/wip-16285-expand-fail-drewhk

=str: #16285: Expand should not drop elements on completion
Merge pull request #16404 from ktoso/str-verify-headsubscriber-ktoso

+str #15804 blackbox verification for head subscriber
=str #15152 operation transformations