
theorem Th539:
Sum digits(2100,10) = 3
proof
  thus Sum digits(2100,10) = Sum <%0,0,1%>+Sum <%2%> by Th538,AFINSQ_2:55
  .= Sum <%0,0%> + Sum <%1%> + Sum <%2%> by AFINSQ_2:55
  .= (0 + 0) + Sum <%1%> + Sum <%2%> by AFINSQ_2:54
  .= (0 + 0 + 1) + Sum <%2%> by AFINSQ_2:53
  .= 1 + 2 by AFINSQ_2:53
  .= 3;
end;
