
theorem Th863:
Sum digits(1716,10) = 15
proof
  thus Sum digits(1716,10) = Sum <%6,1,7%>+Sum <%1%> by Th862,AFINSQ_2:55
  .= Sum <%6,1%> + Sum <%7%> + Sum <%1%> by AFINSQ_2:55
  .= (6 + 1) + Sum <%7%> + Sum <%1%> by AFINSQ_2:54
  .= (6 + 1 + 7) + Sum <%1%> by AFINSQ_2:53
  .= 14 + 1 by AFINSQ_2:53
  .= 15;
end;
