>>> import regexlint
>>> regexlint.Regex().get_parse_tree("(?-i:a)")
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/home/jean/repos/regexlint/regexlint/parser.py", line 377, in get_parse_tree
return cls._get_parse_tree(s, flags, effective_flags)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/home/jean/repos/regexlint/regexlint/parser.py", line 474, in _get_parse_tree
c = open_stack[-1].children.pop()
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
IndexError: pop from empty list
