Lena is developing a small utility for the team's debugging dashboard to verify if a combined activity log correctly merges events from two different servers. The tool needs to check if the final sequence preserves the internal order of both original streams, ensuring no events were dropped or rearranged during the merge. Mark reviewed her draft and suggested she optimize the logic to handle longer sequences more efficiently without using extra memory. For example, if one server sent 'mail' and another sent 'sent', a log showing 'masenilt' would be flagged as a valid combination, while 'linesamt' would be rejected.