thus Sum (digits (1210,10)) = (Sum <%0,1,2%>) + (Sum <%1%>) by Th820, AFINSQ_2:55
.= ((Sum <%0,1%>) + (Sum <%2%>)) + (Sum <%1%>) by AFINSQ_2:55
.= ((0 + 1) + (Sum <%2%>)) + (Sum <%1%>) by AFINSQ_2:54
.= ((0 + 1) + 2) + (Sum <%1%>) by AFINSQ_2:53
.= 3 + 1 by AFINSQ_2:53
.= 4 ; :: thesis: verum