do input""a$ b=len(a$) if!b end s=0 j=3 f=b=17 for i=1to b-4j=4-j if!f j=i c$=mid$(a$,i+4,1) if"?"=c$k=j:n=i s=s+j*(val(c$)+10*c$="X")next for i=0to 9if!mod(i*k+s,11-f)u=i next mid$(a$,n+4)=str$(u) ?a$loop
Note that non-ascii characters in the above source code will be escaped (such as \x9f).