reserve n for Nat;

theorem bag2:
for R being domRing,
    a being Element of R holds card BRoots rpoly(1,a) = 1
proof
let R be domRing, a be Element of R;
BRoots rpoly(1,a) = BRoots <%-a, 1.R%> by repr
                 .= ({a}, 1)-bag by UPROOTS:54;
then card BRoots rpoly(1,a) = card {a} by UPROOTS:13;
hence thesis by CARD_1:30;
end;
