
theorem Th531:
Sum digits(2020,10) = 4
proof
  thus Sum digits(2020,10) = Sum <%0,2,0%>+Sum <%2%> by Th530,AFINSQ_2:55
  .= Sum <%0,2%> + Sum <%0%> + Sum <%2%> by AFINSQ_2:55
  .= (0 + 2) + Sum <%0%> + Sum <%2%> by AFINSQ_2:54
  .= (0 + 2 + 0) + Sum <%2%> by AFINSQ_2:53
  .= 2 + 2 by AFINSQ_2:53
  .= 4;
end;
