
theorem Th19:
Sum digits(20,10) = 2
proof
  thus Sum digits(20,10) = Sum <%0%>+Sum <%2%> by Th18,AFINSQ_2:55
  .= 0 + Sum <%2%> by AFINSQ_2:53
  .= 2 by AFINSQ_2:53;
end;
