
theorem Th1224:
Sum digits(2775,10) = 21
proof
  thus Sum digits(2775,10) = Sum <%5,7,7%>+Sum <%2%> by Th1223,AFINSQ_2:55
  .= Sum <%5,7%> + Sum <%7%> + Sum <%2%> by AFINSQ_2:55
  .= (5 + 7) + Sum <%7%> + Sum <%2%> by AFINSQ_2:54
  .= (5 + 7 + 7) + Sum <%2%> by AFINSQ_2:53
  .= 19 + 2 by AFINSQ_2:53
  .= 21;
end;
