Creating a sequence variable for crossover studies in SAS datastep

I have the following dataset from a crossover design study with participant_id, treatment_arm, and date_of_treatment as follows:

So for participant_id 1, based on the order of the date_of_treatment, the sequence would be ABC. For participant_id 2, it would be CBA.

Based on the above, I want to create column seq as follows:

How do I go about creating the column using the 3 variables participant_id, treatment_arm, and date_of_treatment in datastep?

You could use a double DoW Loop

data want;
   do until (last.participant_id); 
      set have;   
      length seq :$3.;
      by participant_id;
      seq = cats(seq, treatment_arm);        
   end; 
   
   do until (last.participant_id);
      set have;
      by participant_id;
      output;
   end;
run;

Remember to change the length of seq should there be more than 3 treatments for each participant.

participant_id     treatment_arm     date_of_treatment     seq    
      1                 A                01JAN2022         ABC
      1                 B                02JAN2022         ABC
      1                 C                03JAN2022         ABC
      2                 C                04JAN2022         CBA
      2                 B                05JAN2022         CBA
      2                 A                06JAN2022         CBA