configuring ffl for ciforth

https://github.com/irdvo/ffl

ffl takes no consideration for case sensitive Forth's.
So it took some more time, but I made same progress.
However I got stuck upon the following:
---------------------------------

[UNDEFINED] nil<>? [IF]

0 nil= [IF]
: nil<>?    ( addr -- false | addr true = If addr is nil, then return false, else return address with true )
  state @ IF
    postpone ?dup
  ELSE
    ?dup
  THEN
; immediate
[ELSE]
: nil<>?
  dup nil<> IF
    true
  ELSE
    drop
    false
  THEN
;
[THEN

[THEN]
--------------------------------------------------

This combines a number of Forth stuff that I hate:
- Defining words by inspecting if it is undefined.
- an immediate word that it is smart
- a word that has variable output
- it contains ?DUP (bleh)
- it contains POSTPONE even postponing ?DUP

I can't progress because van Oudheusden uses this all over the place,
literally in every file, and I can't compile this piece of code.

********** PROBLEM ************************
ciforth don't know the word `` [THEN ''.
*******************************************
[I checked it and it is really in irho/ffl.]

What gives? 40 years of software maintenance have taught me to never
ever try to remove a defect if the specification of the module
is not clear. You can't properly test it without a specification,
you know.

For what it is worth gforth gives the message:
[THEN
:1: Undefined word

[THEN<<<

Can you help me?
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