
theorem Th669:
Sum digits(3400,10) = 7
proof
  thus Sum digits(3400,10) = Sum <%0,0,4%>+Sum <%3%> by Th668,AFINSQ_2:55
  .= Sum <%0,0%> + Sum <%4%> + Sum <%3%> by AFINSQ_2:55
  .= (0 + 0) + Sum <%4%> + Sum <%3%> by AFINSQ_2:54
  .= (0 + 0 + 4) + Sum <%3%> by AFINSQ_2:53
  .= 4 + 3 by AFINSQ_2:53
  .= 7;
end;
