How to realize the functions like (gen sum) and (egen sum) for string variables by group?

In the sample data, B is what I want to get from A (something like egen sum). I also want to genarate a variable as the sum of the first through jth observations on A (something like gen sum).

Code:

* Example generated by -dataex-. To install: ssc install dataex
clear
input str54 A float tempid str65 B
"negAffect5"            1 "negAffect5 posAffect4 negAffect2 negAffect3"                      
"posAffect4"            1 "negAffect5 posAffect4 negAffect2 negAffect3"                      
"negAffect2 negAffect3" 1 "negAffect5 posAffect4 negAffect2 negAffect3"                      
"negAffect5"            2 "negAffect5 posAffect4 negAffect2 negAffect3"                      
"posAffect4"            2 "negAffect5 posAffect4 negAffect2 negAffect3"                      
"negAffect2 negAffect3" 2 "negAffect5 posAffect4 negAffect2 negAffect3"                      
"negAffect2 negAffect3" 3 "negAffect2 negAffect3 negAffect2 negAffect5 negAffect3 negAffect4"
"negAffect2 negAffect5" 3 "negAffect2 negAffect3 negAffect2 negAffect5 negAffect3 negAffect4"
"negAffect3 negAffect4" 3 "negAffect2 negAffect3 negAffect2 negAffect5 negAffect3 negAffect4"
end

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