
theorem Th1067:
Sum digits(768,10) = 21
proof
  thus Sum digits(768,10) = Sum <%8,6%>+Sum <%7%> by Th1066,AFINSQ_2:55
  .= (8 + 6) + Sum <%7%> by AFINSQ_2:54
  .= 14 + 7 by AFINSQ_2:53
  .= 21;
end;
