2 setecho.

DEFINE
P0 == [];                                       # empty code
P1 == [2 3 + dup *];                            # non-empty code
P2 == [[1 2 3] [dup *] map];                    # nest data & code
P3 == [ [[1 2][3 4][5] []] [[dup *] map] map ]. # double nesting

DEFINE
    c  ==  concat;
    j2f-f ==
        [ list ]
        [ [[[]] concat] dip [j2f-f] step [[] cons c] concat ]
        [ [] cons [c] cons concat ]
        ifte;
    j2f-forwards == [[]] swap [j2f-f] step.

P0 .
P0 j2f-forwards .
P0 j2f-forwards i .

P1 .
P1 j2f-forwards .
P1 j2f-forwards i .
P1 j2f-forwards i i .

P2 .
P2 j2f-forwards .
P2 j2f-forwards i .
P2 j2f-forwards i i .

P3 .
P3 j2f-forwards .
P3 j2f-forwards i .
P3 j2f-forwards i i .

DEFINE
    s  ==  swoncat;
    j2f-r ==
        [ list ]
        [ [[swons] swoncat] dip [j2f-r] step [] swons ]
        [ [] cons [s] cons swoncat ]
        ifte;
    j2f-reverse == [] swap [j2f-r] step [] swons.


P0 .
P0 j2f-reverse .
P0 j2f-reverse i .

P1 .
P1 j2f-reverse .
P1 j2f-reverse i .
P1 j2f-reverse i i .

P2 .
P2 j2f-reverse .
P2 j2f-reverse i .
P2 j2f-reverse i i .

P3 .
P3 j2f-reverse .
P3 j2f-reverse i .
P3 j2f-reverse i i .

                # Double flattening - an exercise in Joy obfuscation:
P3 j2f-forwards j2f-reverse .
                # partial de-obfuscation  -  by Joy itself:
P3 j2f-forwards j2f-reverse i .
                # full de-obfuscation  -  again by Joy:
P3 j2f-forwards j2f-reverse i i .
                # final run of de-obfuscated original:
P3 j2f-forwards j2f-reverse i i i .