
theorem Th90:
Sum digits(209,10) = 11
proof
  thus Sum digits(209,10) = Sum <%9,0%>+Sum <%2%> by Th89,AFINSQ_2:55
  .= (9 + 0) + Sum <%2%> by AFINSQ_2:54
  .= 9 + 2 by AFINSQ_2:53
  .= 11;
end;
