
theorem Th1200:
Sum digits(2375,10) = 17
proof
  thus Sum digits(2375,10) = Sum <%5,7,3%>+Sum <%2%> by Th1199,AFINSQ_2:55
  .= Sum <%5,7%> + Sum <%3%> + Sum <%2%> by AFINSQ_2:55
  .= (5 + 7) + Sum <%3%> + Sum <%2%> by AFINSQ_2:54
  .= (5 + 7 + 3) + Sum <%2%> by AFINSQ_2:53
  .= 15 + 2 by AFINSQ_2:53
  .= 17;
end;
