
theorem Th1260:
Sum digits(3375,10) = 18
proof
  thus Sum digits(3375,10) = Sum <%5,7,3%>+Sum <%3%> by Th1259,AFINSQ_2:55
  .= Sum <%5,7%> + Sum <%3%> + Sum <%3%> by AFINSQ_2:55
  .= (5 + 7) + Sum <%3%> + Sum <%3%> by AFINSQ_2:54
  .= (5 + 7 + 3) + Sum <%3%> by AFINSQ_2:53
  .= 15 + 3 by AFINSQ_2:53
  .= 18;
end;
