thus Sum (digits (3980,10)) =
(Sum <%0,8,9%>) + (Sum <%3%>)
by Th726, AFINSQ_2:55
.=
((Sum <%0,8%>) + (Sum <%9%>)) + (Sum <%3%>)
by AFINSQ_2:55
.=
((0 + 8) + (Sum <%9%>)) + (Sum <%3%>)
by AFINSQ_2:54
.=
((0 + 8) + 9) + (Sum <%3%>)
by AFINSQ_2:53
.=
17 + 3
by AFINSQ_2:53
.=
20
; verum