thus Sum (digits (2140,10)) = (Sum <%0,4,1%>) + (Sum <%2%>) by Th542, AFINSQ_2:55
.= ((Sum <%0,4%>) + (Sum <%1%>)) + (Sum <%2%>) by AFINSQ_2:55
.= ((0 + 4) + (Sum <%1%>)) + (Sum <%2%>) by AFINSQ_2:54
.= ((0 + 4) + 1) + (Sum <%2%>) by AFINSQ_2:53
.= 5 + 2 by AFINSQ_2:53
.= 7 ; :: thesis: verum