
theorem Th607:
Sum digits(2780,10) = 17
proof
  thus Sum digits(2780,10) = Sum <%0,8,7%>+Sum <%2%> by Th606,AFINSQ_2:55
  .= Sum <%0,8%> + Sum <%7%> + Sum <%2%> by AFINSQ_2:55
  .= (0 + 8) + Sum <%7%> + Sum <%2%> by AFINSQ_2:54
  .= (0 + 8 + 7) + Sum <%2%> by AFINSQ_2:53
  .= 15 + 2 by AFINSQ_2:53
  .= 17;
end;
