
theorem Th901:
Sum digits(2178,10) = 18
proof
  thus Sum digits(2178,10) = Sum <%8,7,1%>+Sum <%2%> by Th900,AFINSQ_2:55
  .= Sum <%8,7%> + Sum <%1%> + Sum <%2%> by AFINSQ_2:55
  .= (8 + 7) + Sum <%1%> + Sum <%2%> by AFINSQ_2:54
  .= (8 + 7 + 1) + Sum <%2%> by AFINSQ_2:53
  .= 16 + 2 by AFINSQ_2:53
  .= 18;
end;
