
theorem Th855:
Sum digits(1628,10) = 17
proof
  thus Sum digits(1628,10) = Sum <%8,2,6%>+Sum <%1%> by Th854,AFINSQ_2:55
  .= Sum <%8,2%> + Sum <%6%> + Sum <%1%> by AFINSQ_2:55
  .= (8 + 2) + Sum <%6%> + Sum <%1%> by AFINSQ_2:54
  .= (8 + 2 + 6) + Sum <%1%> by AFINSQ_2:53
  .= 16 + 1 by AFINSQ_2:53
  .= 17;
end;
