thus Sum (digits (1012,10)) = (Sum <%2,1,0%>) + (Sum <%1%>) by Th804, AFINSQ_2:55
.= ((Sum <%2,1%>) + (Sum <%0%>)) + (Sum <%1%>) by AFINSQ_2:55
.= ((2 + 1) + (Sum <%0%>)) + (Sum <%1%>) by AFINSQ_2:54
.= ((2 + 1) + 0) + (Sum <%1%>) by AFINSQ_2:53
.= 3 + 1 by AFINSQ_2:53
.= 4 ; :: thesis: verum