
theorem Th2:
  for f being FinSequence,i being Nat st i>=len f holds f/^i={}
proof
  let f be FinSequence,i be Nat;
  assume
A1: i>=len f;
  per cases by A1,XXREAL_0:1;
  suppose i>len f;
    hence thesis by RFINSEQ:def 1;
  end;
  suppose
A2: i=len f;
    then len (f/^i)=len f -i by RFINSEQ:def 1
      .=0 by A2;
    hence thesis;
  end;
end;
