thus Sum (digits (2112,10)) = (Sum <%2,1,1%>) + (Sum <%2%>) by Th894, AFINSQ_2:55
.= ((Sum <%2,1%>) + (Sum <%1%>)) + (Sum <%2%>) by AFINSQ_2:55
.= ((2 + 1) + (Sum <%1%>)) + (Sum <%2%>) by AFINSQ_2:54
.= ((2 + 1) + 1) + (Sum <%2%>) by AFINSQ_2:53
.= 4 + 2 by AFINSQ_2:53
.= 6 ; :: thesis: verum