
theorem Th843:
Sum digits(1474,10) = 16
proof
  thus Sum digits(1474,10) = Sum <%4,7,4%>+Sum <%1%> by Th842,AFINSQ_2:55
  .= Sum <%4,7%> + Sum <%4%> + Sum <%1%> by AFINSQ_2:55
  .= (4 + 7) + Sum <%4%> + Sum <%1%> by AFINSQ_2:54
  .= (4 + 7 + 4) + Sum <%1%> by AFINSQ_2:53
  .= 15 + 1 by AFINSQ_2:53
  .= 16;
end;
