
theorem Th551:
Sum digits(2220,10) = 6
proof
  thus Sum digits(2220,10) = Sum <%0,2,2%>+Sum <%2%> by Th550,AFINSQ_2:55
  .= Sum <%0,2%> + Sum <%2%> + Sum <%2%> by AFINSQ_2:55
  .= (0 + 2) + Sum <%2%> + Sum <%2%> by AFINSQ_2:54
  .= (0 + 2 + 2) + Sum <%2%> by AFINSQ_2:53
  .= 4 + 2 by AFINSQ_2:53
  .= 6;
end;
