Resilient Path Navigation

A drone processes direction commands: NORTH, SOUTH, EAST, WEST. A command cancels the immediate previous command if they are opposites (e.g., NORTH cancels SOUTH). However, a 'FIX' command makes all commands currently in the history permanent, meaning they can no longer be cancelled by future opposites. Input is one command per line. Output the final list of uncancelled commands. Example: NORTH FIX SOUTH EAST WEST Output: NORTH SOUTH