
theorem Th960:
Sum digits(575,10) = 17
proof
  thus Sum digits(575,10) = Sum <%5,7%>+Sum <%5%> by Th959,AFINSQ_2:55
  .= (5 + 7) + Sum <%5%> by AFINSQ_2:54
  .= 12 + 5 by AFINSQ_2:53
  .= 17;
end;
