
theorem Th641:
Sum digits(3120,10) = 6
proof
  thus Sum digits(3120,10) = Sum <%0,2,1%>+Sum <%3%> by Th640,AFINSQ_2:55
  .= Sum <%0,2%> + Sum <%1%> + Sum <%3%> by AFINSQ_2:55
  .= (0 + 2) + Sum <%1%> + Sum <%3%> by AFINSQ_2:54
  .= (0 + 2 + 1) + Sum <%3%> by AFINSQ_2:53
  .= 3 + 3 by AFINSQ_2:53
  .= 6;
end;
