The sequence is generated by following this pseudo-code:

A:  The first term is zero.
    Repeatedly apply:
        If the last term is *new* to the sequence so far then:
B:          The next term is zero.
        Otherwise:
C:          The next term is how far back this last term occured previously.



Example

Using A:

0
Using B:

0 0
Using C:

0 0 1
Using B:

0 0 1 0
Using C: (zero last occurred two steps back - before the one)

0 0 1 0 2
Using B:

0 0 1 0 2 0
Using C: (two last occurred two steps back - before the zero)

0 0 1 0 2 0 2 2
Using C: (two last occurred one step back)

0 0 1 0 2 0 2 2 1
Using C: (one last appeared six steps back)

0 0 1 0 2 0 2 2 1 6
...



Task

 Create a function/procedure/method/subroutine/... to generate the Van Eck sequence of numbers.
 Use it to display here, on this page:
 The first ten terms of the sequence.
 Terms 991 - to - 1000 of the sequence.


References

 Don't Know (the Van Eck Sequence) - Numberphile video.
 Wikipedia Article: Van Eck's Sequence.
  OEIS sequence: A181391.


int Seq(1001), Back, N, M, Term;
 
func New;       \Return 'true' if last term is new to sequence
[for Back:= N-2 downto 1 do
        if Seq(Back) = Seq(N-1) then return false;
return true;
];
 
func VanEck;    \Return term of Van Eck sequence
[Seq(N):= if New then 0 else N-Back-1;
N:= N+1;
return Seq(N-1);
];
 
[N:= 1;
for M:= 1 to 1000 do
    [Term:= VanEck;
    if M<=10 or M>=991 then
        [IntOut(0, Term);
        if M=10 or M=1000 then Crlf(0) else Text(0, ", ");
        ];
    ];
]