
theorem Th1320:
Sum digits(4275,10) = 18
proof
  thus Sum digits(4275,10) = Sum <%5,7,2%>+Sum <%4%> by Th1319,AFINSQ_2:55
  .= Sum <%5,7%> + Sum <%2%> + Sum <%4%> by AFINSQ_2:55
  .= (5 + 7) + Sum <%2%> + Sum <%4%> by AFINSQ_2:54
  .= (5 + 7 + 2) + Sum <%4%> by AFINSQ_2:53
  .= 14 + 4 by AFINSQ_2:53
  .= 18;
end;
