thus Sum (digits (2156,10)) = (Sum <%6,5,1%>) + (Sum <%2%>) by Th898, AFINSQ_2:55
.= ((Sum <%6,5%>) + (Sum <%1%>)) + (Sum <%2%>) by AFINSQ_2:55
.= ((6 + 5) + (Sum <%1%>)) + (Sum <%2%>) by AFINSQ_2:54
.= ((6 + 5) + 1) + (Sum <%2%>) by AFINSQ_2:53
.= 12 + 2 by AFINSQ_2:53
.= 14 ; :: thesis: verum