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.


get "libhdr"
 
let start() be
$(  let eck = vec 999
 
    for i = 0 to 999 do eck!i := 0
    for i = 0 to 998 do
        for j = i-1 to 0 by -1 do
            if eck!i = eck!j then
            $(  eck!(i+1) := i-j
                break
            $)
 
    for i = 0 to 9 do writed(eck!i, 4)
    wrch('*N')
    for i = 990 to 999 do writed(eck!i, 4)
    wrch('*N')
$)