thus Sum (digits (2760,10)) = (Sum <%0,6,7%>) + (Sum <%2%>) by Th604, AFINSQ_2:55
.= ((Sum <%0,6%>) + (Sum <%7%>)) + (Sum <%2%>) by AFINSQ_2:55
.= ((0 + 6) + (Sum <%7%>)) + (Sum <%2%>) by AFINSQ_2:54
.= ((0 + 6) + 7) + (Sum <%2%>) by AFINSQ_2:53
.= 13 + 2 by AFINSQ_2:53
.= 15 ; :: thesis: verum