def keys(self):
return self._keys(self.parselet_tree)<EOL>
Return a list of 1st level keys of the output data model

>>> import parslepy
>>> rules = {
...     "headingcss": "#main",
...     "headingxpath": "//h1[@id='main']"
... }
>>> p = parslepy.Parselet(rules)
>>> sorted(p.keys())
['headingcss', 'headingxpath']