
theorem Th871:
Sum digits(1826,10) = 17
proof
  thus Sum digits(1826,10) = Sum <%6,2,8%>+Sum <%1%> by Th870,AFINSQ_2:55
  .= Sum <%6,2%> + Sum <%8%> + Sum <%1%> by AFINSQ_2:55
  .= (6 + 2) + Sum <%8%> + Sum <%1%> by AFINSQ_2:54
  .= (6 + 2 + 8) + Sum <%1%> by AFINSQ_2:53
  .= 16 + 1 by AFINSQ_2:53
  .= 17;
end;
