
theorem Th541:
Sum digits(2120,10) = 5
proof
  thus Sum digits(2120,10) = Sum <%0,2,1%>+Sum <%2%> by Th540,AFINSQ_2:55
  .= Sum <%0,2%> + Sum <%1%> + Sum <%2%> by AFINSQ_2:55
  .= (0 + 2) + Sum <%1%> + Sum <%2%> by AFINSQ_2:54
  .= (0 + 2 + 1) + Sum <%2%> by AFINSQ_2:53
  .= 3 + 2 by AFINSQ_2:53
  .= 5;
end;
