say a transfer rule outputs
^Imp<SV><@+FAUXV>{^ij<vblex><neg><imp><p2><sg>$} $
(due to <chunk>...<b/></chunk> which the DTD allows)
then postchunk gives
$ echo '^Imp<SV><@+FAUXV>{^ij<vblex><neg><imp><p2><sg>$} $ ^adv<ADV><@←ADVL>{^olkese<adv>$}$'|apertium-postchunk apertium-sme-sma.sme-sma.t5x sme-sma.t5x.bin
^Ij<vblex><neg><imp><p2><sg>$} $ ^adv<ADV><@←ADVL>{^olkese<adv>$
and hfst-proc crashes on that.
The DTD should disallow it, or interchunk should fix it behind the scenes, or postchunk should output correctly even if the input is weird.