
theorem Th491:
Sum digits(1620,10) = 9
proof
  thus Sum digits(1620,10) = Sum <%0,2,6%>+Sum <%1%> by Th490,AFINSQ_2:55
  .= Sum <%0,2%> + Sum <%6%> + Sum <%1%> by AFINSQ_2:55
  .= (0 + 2) + Sum <%6%> + Sum <%1%> by AFINSQ_2:54
  .= (0 + 2 + 6) + Sum <%1%> by AFINSQ_2:53
  .= 8 + 1 by AFINSQ_2:53
  .= 9;
end;
