
theorem Th1026:
Sum digits(1426,10) = 13
proof
  thus Sum digits(1426,10) = Sum <%6,2,4%>+Sum <%1%> by Th1025,AFINSQ_2:55
  .= Sum <%6,2%> + Sum <%4%> + Sum <%1%> by AFINSQ_2:55
  .= (6 + 2) + Sum <%4%> + Sum <%1%> by AFINSQ_2:54
  .= (6 + 2 + 4) + Sum <%1%> by AFINSQ_2:53
  .= 12 + 1 by AFINSQ_2:53
  .= 13;
end;
