reserve X,G for RealNormSpace-Sequence,
          Y for RealNormSpace;
reserve f for MultilinearOperator of X,Y;

theorem DCARXX: ::: should be proven before
  dom carr X = dom X
  proof
    dom carr X = Seg len carr X by FINSEQ_1:def 3;
    hence dom carr X = Seg len X by PRVECT_1:def 11
    .= dom X by FINSEQ_1:def 3;
  end;
