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.


            NORMAL MODE IS INTEGER
            DIMENSION E(1000)
            E(0)=0
            THROUGH L1, FOR I=0, 1, I.GE.1000
            THROUGH L2, FOR J=I-1, -1, J.L.0
            WHENEVER E(J).E.E(I)
                E(I+1) = I-J
                TRANSFER TO L1
            END OF CONDITIONAL
L2          CONTINUE
            E(I+1)=0
L1          CONTINUE
            THROUGH S, FOR I=0, 1, I.GE.10
S           PRINT FORMAT FMT, I, E(I), I+990, E(I+990)
            VECTOR VALUES FMT = $2HE(,I3,2H)=,I3,S5,2HE(,I3,2H)=,I3*$
            END OF PROGRAM 