
theorem Th917:
Sum digits(2376,10) = 18
proof
  thus Sum digits(2376,10) = Sum <%6,7,3%>+Sum <%2%> by Th916,AFINSQ_2:55
  .= Sum <%6,7%> + Sum <%3%> + Sum <%2%> by AFINSQ_2:55
  .= (6 + 7) + Sum <%3%> + Sum <%2%> by AFINSQ_2:54
  .= (6 + 7 + 3) + Sum <%2%> by AFINSQ_2:53
  .= 16 + 2 by AFINSQ_2:53
  .= 18;
end;
