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.


      program VanEck
      implicit none
      integer eck(1000), i, j
 
      eck(1) = 0
      do 20 i=1, 999
          do 10 j=i-1, 1, -1
              if (eck(i) .eq. eck(j)) then
                  eck(i+1) = i-j
                  go to 20
              end if
 10       continue
          eck(i+1) = 0
 20   continue
 
      do 30 i=1, 10
 30       write (*,'(I4)',advance='no') eck(i)
      write (*,*)
 
      do 40 i=991, 1000
 40       write (*,'(I4)',advance='no') eck(i)
      write (*,*)
 
      end program