
theorem Th899:
Sum digits(2156,10) = 14
proof
  thus Sum digits(2156,10) = Sum <%6,5,1%>+Sum <%2%> by Th898,AFINSQ_2:55
  .= Sum <%6,5%> + Sum <%1%> + Sum <%2%> by AFINSQ_2:55
  .= (6 + 5) + Sum <%1%> + Sum <%2%> by AFINSQ_2:54
  .= (6 + 5 + 1) + Sum <%2%> by AFINSQ_2:53
  .= 12 + 2 by AFINSQ_2:53
  .= 14;
end;
