
theorem Th74:
Sum digits(121,10) = 4
proof
  thus Sum digits(121,10) = Sum <%1,2%>+Sum <%1%> by Th73,AFINSQ_2:55
  .= (1 + 2) + Sum <%1%> by AFINSQ_2:54
  .= 3 + 1 by AFINSQ_2:53
  .= 4;
end;
