
theorem Th883:
Sum digits(1958,10) = 23
proof
  thus Sum digits(1958,10) = Sum <%8,5,9%>+Sum <%1%> by Th882,AFINSQ_2:55
  .= Sum <%8,5%> + Sum <%9%> + Sum <%1%> by AFINSQ_2:55
  .= (8 + 5) + Sum <%9%> + Sum <%1%> by AFINSQ_2:54
  .= (8 + 5 + 9) + Sum <%1%> by AFINSQ_2:53
  .= 22 + 1 by AFINSQ_2:53
  .= 23;
end;
