code: public boolean deregister(Invocation invocation) {
          if (!deactivate(invocation.op)) {
              return false;
          }
          invocations.remove(invocation.op.getCallId());
          callIdSequence.complete();
          return true;
      }
docs: Deregisters an invocation. If the associated operation is inactive, takes no action and returns {@code false}.
  This ensures the idempotency of deregistration.
  
  @param invocation The Invocation to deregister.
  @return {@code true} if this call deregistered the invocation; {@code false} if the invocation wasn't registered