thus Sum (digits (2120,10)) = (Sum <%0,2,1%>) + (Sum <%2%>) by Th540, AFINSQ_2:55
.= ((Sum <%0,2%>) + (Sum <%1%>)) + (Sum <%2%>) by AFINSQ_2:55
.= ((0 + 2) + (Sum <%1%>)) + (Sum <%2%>) by AFINSQ_2:54
.= ((0 + 2) + 1) + (Sum <%2%>) by AFINSQ_2:53
.= 3 + 2 by AFINSQ_2:53
.= 5 ; :: thesis: verum