reserve a,b,i,j,k,l,m,n for Nat;

theorem FFE:
  for f be FinSequence holds f,(Rev f) are_fiberwise_equipotent
  proof
    let f be FinSequence;
    A1: f = (Rev f)*(Rev (idseq len f)) by REV;
    (Rev (idseq len f)) is Permutation of dom (Rev f) by RFP;
    hence thesis by A1, RFINSEQ:4;
  end;
