thus Sum (digits (1892,10)) = (Sum <%2,9,8%>) + (Sum <%1%>) by Th876, AFINSQ_2:55
.= ((Sum <%2,9%>) + (Sum <%8%>)) + (Sum <%1%>) by AFINSQ_2:55
.= ((2 + 9) + (Sum <%8%>)) + (Sum <%1%>) by AFINSQ_2:54
.= ((2 + 9) + 8) + (Sum <%1%>) by AFINSQ_2:53
.= 19 + 1 by AFINSQ_2:53
.= 20 ; :: thesis: verum