这题怎么算呢？

set talk off
clear
accept"请任意输入一串字符:"to p
l=len(p)
c=""
fori=1tol
   A=substr(p,i,1)
   do case
case asc(A)>=asc("a") and asc(A)<=asc("u") or  asc(A)>=asc("a") and asc(A)<=asc("U")
    a=chr(asc(a)+5)
case asc(A)>=asc("v") and asc(A)<=asc("z") or asc(A)>=asc("V") and asc (A)<=asc("Z")
A=chr(asc(a)-21)
endfor
?P+""+c
set talk on
那个朋友解释下。谢谢了

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