
theorem Th877:
Sum digits(1892,10) = 20
proof
  thus Sum digits(1892,10) = Sum <%2,9,8%>+Sum <%1%> by Th876,AFINSQ_2:55
  .= Sum <%2,9%> + Sum <%8%> + Sum <%1%> by AFINSQ_2:55
  .= (2 + 9) + Sum <%8%> + Sum <%1%> by AFINSQ_2:54
  .= (2 + 9 + 8) + Sum <%1%> by AFINSQ_2:53
  .= 19 + 1 by AFINSQ_2:53
  .= 20;
end;
