thus Sum (digits (1975,10)) = (Sum <%5,7,9%>) + (Sum <%1%>) by Th1175, AFINSQ_2:55
.= ((Sum <%5,7%>) + (Sum <%9%>)) + (Sum <%1%>) by AFINSQ_2:55
.= ((5 + 7) + (Sum <%9%>)) + (Sum <%1%>) by AFINSQ_2:54
.= ((5 + 7) + 9) + (Sum <%1%>) by AFINSQ_2:53
.= 21 + 1 by AFINSQ_2:53
.= 22 ; :: thesis: verum