scope # find elements of the Van Eck Sequence - first term is 0, following
      # terms are 0 if the previous was the first appearance of the element
      #        or how far back in the sequence the last element appeared otherwise

    local proc VanEck( n :: number ) :: sequence # returns the first n elements of the Van Eck sequence
        local constant result := seq(); for i to n do result[ i ] := 0 od;
        local constant pos    := seq(); for i to n + 1 do pos[ i ] := 0 od;
        for i from 2 to n do
            local constant j    := i - 1;
            local constant prev := result[ j ] + 1; # prev is indexed from 1, not 0
            if pos[ prev ] <> 0 then
                # not a new element
                result[ i ] := j - pos[ prev ]
            fi;
            pos[ prev ] := j
        od;
        return result
    end;

    local constant veSeq := VanEck( 1000 ); # construct the first 1000 terms of the sequence
    # show the first and last 10 elements
    for i to 10 do printf( " %d", veSeq[ i ] ) od;
    printf( "\n" );
    for i from size veSeq - 9 to size veSeq do printf( " %d", veSeq[ i ] ) od;
    printf( "\n" )
end