
theorem Th1296:
Sum digits(3975,10) = 24
proof
  thus Sum digits(3975,10) = Sum <%5,7,9%>+Sum <%3%> by Th1295,AFINSQ_2:55
  .= Sum <%5,7%> + Sum <%9%> + Sum <%3%> by AFINSQ_2:55
  .= (5 + 7) + Sum <%9%> + Sum <%3%> by AFINSQ_2:54
  .= (5 + 7 + 9) + Sum <%3%> by AFINSQ_2:53
  .= 21 + 3 by AFINSQ_2:53
  .= 24;
end;
