reserve x,y,z for set;
reserve f,f1,f2,f3 for FinSequence,
  p,p1,p2,p3 for set,
  i,k for Nat;

theorem
  f just_once_values x implies Rev f just_once_values x
proof
  assume
A1: f just_once_values x;
  then
A2: x in rng f by FINSEQ_4:5;
  then not x in rng(f-|x) by FINSEQ_4:37;
  then not x in rng Rev(f-|x) by FINSEQ_5:57;
  then
A3: not x in rng(Rev f |-- x) by A1,Th38;
  x in rng Rev f by A2,FINSEQ_5:57;
  hence thesis by A3,FINSEQ_4:45;
end;
