
theorem Th571:
Sum digits(2420,10) = 8
proof
  thus Sum digits(2420,10) = Sum <%0,2,4%>+Sum <%2%> by Th570,AFINSQ_2:55
  .= Sum <%0,2%> + Sum <%4%> + Sum <%2%> by AFINSQ_2:55
  .= (0 + 2) + Sum <%4%> + Sum <%2%> by AFINSQ_2:54
  .= (0 + 2 + 4) + Sum <%2%> by AFINSQ_2:53
  .= 6 + 2 by AFINSQ_2:53
  .= 8;
end;
