let V be non empty right_complementable Abelian add-associative right_zeroed addLoopStr ; :: thesis: for v being Element of V holds Sum {v} = v
let v be Element of V; :: thesis: Sum {v} = v
( rng <*v*> = {v} & <*v*> is one-to-one & Sum <*v*> = v ) by FINSEQ_1:56, FINSEQ_3:102, RLVECT_1:61;
hence Sum {v} = v by Def4; :: thesis: verum