
theorem Th873:
Sum digits(1848,10) = 21
proof
  thus Sum digits(1848,10) = Sum <%8,4,8%>+Sum <%1%> by Th872,AFINSQ_2:55
  .= Sum <%8,4%> + Sum <%8%> + Sum <%1%> by AFINSQ_2:55
  .= (8 + 4) + Sum <%8%> + Sum <%1%> by AFINSQ_2:54
  .= (8 + 4 + 8) + Sum <%1%> by AFINSQ_2:53
  .= 20 + 1 by AFINSQ_2:53
  .= 21;
end;
