
theorem Th51:
Sum digits(180,10) = 9
proof
  thus Sum digits(180,10) = Sum <%0,8%>+Sum <%1%> by Th50,AFINSQ_2:55
  .= (0 + 8) + Sum <%1%> by AFINSQ_2:54
  .= 8 + 1 by AFINSQ_2:53
  .= 9;
end;
