<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def apply(self, docs, clear=True, parallelism=None, progress_bar=True):
 """Run the MentionExtractor. :Example: To extract mentions from a set of training documents using 4 cores:: mention_extractor.apply(train_docs, parallelism=4) :param docs: Set of documents to extract from. :param clear: Whether or not to clear the existing Mentions beforehand. :type clear: bool :param parallelism: How many threads to use for extraction. This will override the parallelism value used to initialize the MentionExtractor if it is provided. :type parallelism: int :param progress_bar: Whether or not to display a progress bar. The progress bar is measured per document. :type progress_bar: bool """

        super(MentionExtractor, self).apply(
            docs, clear=clear, parallelism=parallelism, progress_bar=progress_bar
        )