
theorem Th867:
Sum digits(1782,10) = 18
proof
  thus Sum digits(1782,10) = Sum <%2,8,7%>+Sum <%1%> by Th866,AFINSQ_2:55
  .= Sum <%2,8%> + Sum <%7%> + Sum <%1%> by AFINSQ_2:55
  .= (2 + 8) + Sum <%7%> + Sum <%1%> by AFINSQ_2:54
  .= (2 + 8 + 7) + Sum <%1%> by AFINSQ_2:53
  .= 17 + 1 by AFINSQ_2:53
  .= 18;
end;
