
theorem Th821:
Sum digits(1210,10) = 4
proof
  thus Sum digits(1210,10) = Sum <%0,1,2%>+Sum <%1%> by Th820,AFINSQ_2:55
  .= Sum <%0,1%> + Sum <%2%> + Sum <%1%> by AFINSQ_2:55
  .= (0 + 1) + Sum <%2%> + Sum <%1%> by AFINSQ_2:54
  .= (0 + 1 + 2) + Sum <%1%> by AFINSQ_2:53
  .= 3 + 1 by AFINSQ_2:53
  .= 4;
end;
