
theorem Th1120:
Sum digits(975,10) = 21
proof
  thus Sum digits(975,10) = Sum <%5,7%>+Sum <%9%> by Th1119,AFINSQ_2:55
  .= (5 + 7) + Sum <%9%> by AFINSQ_2:54
  .= 12 + 9 by AFINSQ_2:53
  .= 21;
end;
