[O] column formulas: Row descriptor -1 used in line ### leads outside table

[andersvi]

Anyone have suggestions about how i can have a column-formula refer to
value in previous row, but ignore the row before the first line?

Something like this:

    | 1 | 4 |   |
    | 2 | 7 |   |
    | 3 | 1 |   |
    #+TBLFM: $2=random(10)::$3=(@-1$2)-$2

Evaluating this returns an error:

  "Row descriptor -1 used in line 547 leads outside table"

I tried using a lisp-expression to dispatch on the row-number, but it
doesnt seem to consider 'if as a special-form, trying to evaluate its
last argument anyhow.

    | 1 | 2 |   |
    | 2 | 6 |   |
    | 3 | 2 |   |
    #+TBLFM: $2=random(10)::$3='(if (< @# 3) 0 (- @-1$2 $2))

but get the same error.

Thanks,

-anders