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.


sub n-van-ecks ($init) {
    $init, -> $i, {
        state %v;
        state $k;
        $k++;
        my $t  = %v{$i}.defined ?? $k - %v{$i} !! 0;
        %v{$i} = $k;
        $t
    } ... *
}
 
for <
    A181391 0
    A171911 1
    A171912 2
    A171913 3
    A171914 4
    A171915 5
    A171916 6
    A171917 7
    A171918 8
> -> $seq, $start {
 
    my @seq = n-van-ecks($start);
 
    # The task
    put qq:to/END/
 
    Van Eck sequence OEIS:$seq; with the first term: $start
            First 10 terms: {@seq[^10]}
    Terms 991 through 1000: {@seq[990..999]}
    END
}