
theorem Th671:
Sum digits(3420,10) = 9
proof
  thus Sum digits(3420,10) = Sum <%0,2,4%>+Sum <%3%> by Th670,AFINSQ_2:55
  .= Sum <%0,2%> + Sum <%4%> + Sum <%3%> by AFINSQ_2:55
  .= (0 + 2) + Sum <%4%> + Sum <%3%> by AFINSQ_2:54
  .= (0 + 2 + 4) + Sum <%3%> by AFINSQ_2:53
  .= 6 + 3 by AFINSQ_2:53
  .= 9;
end;
