
theorem YZ:
for F being Field, a being non zero Element of F holds -((-a)") = a"
proof
let F be Field, a be non zero Element of F;
a" - -((-a)") = a" + -(a") by XZ
             .= 0.F by RLVECT_1:5;
hence thesis by RLVECT_1:21;
end;
