thus Sum (digits (1782,10)) = (Sum <%2,8,7%>) + (Sum <%1%>) by Th866, AFINSQ_2:55
.= ((Sum <%2,8%>) + (Sum <%7%>)) + (Sum <%1%>) by AFINSQ_2:55
.= ((2 + 8) + (Sum <%7%>)) + (Sum <%1%>) by AFINSQ_2:54
.= ((2 + 8) + 7) + (Sum <%1%>) by AFINSQ_2:53
.= 17 + 1 by AFINSQ_2:53
.= 18 ; :: thesis: verum