
theorem Th217:
Sum digits(432,10) = 9
proof
  thus Sum digits(432,10) = Sum <%2,3%>+Sum <%4%> by Th216,AFINSQ_2:55
  .= (2 + 3) + Sum <%4%> by AFINSQ_2:54
  .= 5 + 4 by AFINSQ_2:53
  .= 9;
end;
