(gdb) bt
#0  0x00007f02a38a72df in xmlDictOwns () from /usr/lib/x86_64-linux-gnu/libxml2.so.2
#1  0x00007f02a37c9e73 in xmlFreeNode () from /usr/lib/x86_64-linux-gnu/libxml2.so.2
#2  0x00007f02a3e4fc40 in __pyx_f_4lxml_5etree__removeText (__pyx_v_c_node=<optimized out>) at src/lxml/etree.c:26062
#3  0x00007f02a3e92166 in __pyx_f_4lxml_5etree__setTailText (__pyx_v_c_node=0x562cf0021230, __pyx_v_value=None) at src/lxml/etree.c:26397
#4  0x00007f02a3e922b8 in __pyx_pf_4lxml_5etree_8_Element_4tail_2__set__ (__pyx_v_value=<optimized out>, __pyx_v_self=<optimized out>) at src/lxml/etree.c:56079
#5  __pyx_pw_4lxml_5etree_8_Element_4tail_3__set__ (__pyx_v_value=<optimized out>, __pyx_v_self=<optimized out>) at src/lxml/etree.c:56050
#6  __pyx_setprop_4lxml_5etree_8_Element_tail (o=<optimized out>, v=<optimized out>, x=<optimized out>) at src/lxml/etree.c:13761
#7  0x0000562ceee9797f in _PyObject_GenericSetAttrWithDict (obj=<lxml.etree._Element at remote 0x7f029f6d8950>, name='tail', value=None, dict=0x0)
    at ../Objects/object.c:1512
#8  0x0000562ceee97549 in PyObject_SetAttr (v=<lxml.etree._Element at remote 0x7f029f6d8950>, name='tail', value=None) at ../Objects/object.c:1252
#9  0x0000562ceef2a2b9 in PyEval_EvalFrameEx (
    f=Frame 0x7f029f9bc240, for file /home/mek/specs/bikeshed/bikeshed/htmlhelpers.py, line 415, in childNodes (parentEl=<lxml.etree._Element at remote 0x7f029fa0bb48>, clear=True, skipOddNodes=True, ret=['[', <lxml.etree._Element at remote 0x7f029f6d8950>, '=(Window,Worker), '], c=<lxml.etree._Element at remote 0x7f029f6d8950>),
    throwflag=<optimized out>) at ../Python/ceval.c:2269
#10 0x0000562ceef2710a in PyEval_EvalCodeEx (co=<optimized out>, globals=<optimized out>, locals=<optimized out>, args=<optimized out>, argcount=<optimized out>,
    kws=<optimized out>, kwcount=1, defs=0x7f02a00fc5c0, defcount=2, closure=0x0) at ../Python/ceval.c:3608
#11 0x0000562ceef2ea4e in fast_function (nk=1, na=<optimized out>, n=<optimized out>, pp_stack=0x7ffff6579020, func=<optimized out>) at ../Python/ceval.c:4471
#12 call_function (oparg=<optimized out>, pp_stack=0x7ffff6579020) at ../Python/ceval.c:4396
#13 PyEval_EvalFrameEx (
    f=Frame 0x562cf053e780, for file /home/mek/specs/bikeshed/bikeshed/highlight.py, line 237, in colorizeEl (el=<lxml.etree._Element at remote 0x7f029fa0bb48>, coloredText=<collections.deque at remote 0x7f029ef0bb40>), throwflag=<optimized out>) at ../Python/ceval.c:3013
#14 0x0000562ceef2710a in PyEval_EvalCodeEx (co=<optimized out>, globals=<optimized out>, locals=<optimized out>, args=<optimized out>, argcount=<optimized out>,
    kws=<optimized out>, kwcount=0, defs=0x0, defcount=0, closure=(<cell at remote 0x7f029ee58280>, <cell at remote 0x7f029ee58520>)) at ../Python/ceval.c:3608
#15 0x0000562ceef2ef5e in fast_function (nk=0, na=<optimized out>, n=<optimized out>, pp_stack=0x7ffff6579200, func=<optimized out>) at ../Python/ceval.c:4471
#16 call_function (oparg=<optimized out>, pp_stack=0x7ffff6579200) at ../Python/ceval.c:4396
#17 PyEval_EvalFrameEx (
    f=Frame 0x562ceffa1570, for file /home/mek/specs/bikeshed/bikeshed/highlight.py, line 267, in mergeHighlighting (el=<lxml.etree._Element at remote 0x7f029fa0bb48>, coloredText=<collections.deque at remote 0x7f029ef0bb40>), throwflag=<optimized out>) at ../Python/ceval.c:3013
#18 0x0000562ceef2710a in PyEval_EvalCodeEx (co=<optimized out>, globals=<optimized out>, locals=<optimized out>, args=<optimized out>, argcount=<optimized out>,
    kws=<optimized out>, kwcount=0, defs=0x0, defcount=0, closure=0x0) at ../Python/ceval.c:3608
#19 0x0000562ceef2ef5e in fast_function (nk=0, na=<optimized out>, n=<optimized out>, pp_stack=0x7ffff65793e0, func=<optimized out>) at ../Python/ceval.c:4471
#20 call_function (oparg=<optimized out>, pp_stack=0x7ffff65793e0) at ../Python/ceval.c:4396
#21 PyEval_EvalFrameEx (
    f=Frame 0x562cf0540900, for file /home/mek/specs/bikeshed/bikeshed/highlight.py, line 133, in highlightEl (el=<lxml.etree._Element at remote 0x7f029fa0bb48>, lang=u'webidl', text=u'[Exposed=(Window,Worker), SecureContext]\ninterface FileSystemWriter {\n  Promise<void> write(unsigned long long position, (BufferSource or Blob or USVString) data);\n  WritableStream asWritableStream();\n  Promise<void> truncate(unsigned long long size);\n  Promise<void> close();\n};\n', coloredText=<collections.d