Accessing a parse tree

[Clarendon]

Thank you very much for this information. It seems to point me to the
right direction. However, I do not fully understand the flatten
function and its output. Some indices seem to be inaccurate. I tried
to find this function at nltk.tree.Tree.flatten, but it returns a
flattened tree, not a tuple.

So your flatten function must be a different one, and it's not one of
the builtins, either. Could you tell me where I can find the
documentation about this flatten function?