
theorem Th1220:
Sum digits(2725,10) = 16
proof
  thus Sum digits(2725,10) = Sum <%5,2,7%>+Sum <%2%> by Th1219,AFINSQ_2:55
  .= Sum <%5,2%> + Sum <%7%> + Sum <%2%> by AFINSQ_2:55
  .= (5 + 2) + Sum <%7%> + Sum <%2%> by AFINSQ_2:54
  .= (5 + 2 + 7) + Sum <%2%> by AFINSQ_2:53
  .= 14 + 2 by AFINSQ_2:53
  .= 16;
end;
