
theorem Th421:
Sum digits(920,10) = 11
proof
  thus Sum digits(920,10) = Sum <%0,2%>+Sum <%9%> by Th420,AFINSQ_2:55
  .= (0 + 2) + Sum <%9%> by AFINSQ_2:54
  .= 2 + 9 by AFINSQ_2:53
  .= 11;
end;
