
theorem Th471:
Sum digits(1420,10) = 7
proof
  thus Sum digits(1420,10) = Sum <%0,2,4%>+Sum <%1%> by Th470,AFINSQ_2:55
  .= Sum <%0,2%> + Sum <%4%> + Sum <%1%> by AFINSQ_2:55
  .= (0 + 2) + Sum <%4%> + Sum <%1%> by AFINSQ_2:54
  .= (0 + 2 + 4) + Sum <%1%> by AFINSQ_2:53
  .= 6 + 1 by AFINSQ_2:53
  .= 7;
end;
