
theorem Th507:
Sum digits(1780,10) = 16
proof
  thus Sum digits(1780,10) = Sum <%0,8,7%>+Sum <%1%> by Th506,AFINSQ_2:55
  .= Sum <%0,8%> + Sum <%7%> + Sum <%1%> by AFINSQ_2:55
  .= (0 + 8) + Sum <%7%> + Sum <%1%> by AFINSQ_2:54
  .= (0 + 8 + 7) + Sum <%1%> by AFINSQ_2:53
  .= 15 + 1 by AFINSQ_2:53
  .= 16;
end;
