return the text of the given EDU subtree, with '_!'-delimiters removed.

def get_edu_text(text_subtree):
    """return the text of the given EDU subtree, with '_!'-delimiters removed."""
    assert text_subtree.label() == 'text', "text_subtree: {}".format(text_subtree)
    edu_str = u' '.join(word for word in text_subtree.leaves())
    return re.sub('_!(.*?)_!', '\g<1>', edu_str)