thus Sum (digits (1220,10)) = (Sum <%0,2,2%>) + (Sum <%1%>) by Th450, AFINSQ_2:55
.= ((Sum <%0,2%>) + (Sum <%2%>)) + (Sum <%1%>) by AFINSQ_2:55
.= ((0 + 2) + (Sum <%2%>)) + (Sum <%1%>) by AFINSQ_2:54
.= ((0 + 2) + 2) + (Sum <%1%>) by AFINSQ_2:53
.= 4 + 1 by AFINSQ_2:53
.= 5 ; :: thesis: verum