
theorem Th767:
Sum digits(528,10) = 15
proof
  thus Sum digits(528,10) = Sum <%8,2%>+Sum <%5%> by Th766,AFINSQ_2:55
  .= (8 + 2) + Sum <%5%> by AFINSQ_2:54
  .= 10 + 5 by AFINSQ_2:53
  .= 15;
end;
