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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def index_file(self, f, overwrite=False):
 """Index metadata for the specified file. Args: f (BIDSFile, str):
 A BIDSFile or path to an indexed file. overwrite (bool):
 If True, forces reindexing of the file even if an entry already exists. """

        if isinstance(f, six.string_types):
            f = self.layout.get_file(f)

        if f.path in self.file_index and not overwrite:
            return

        if 'suffix' not in f.entities:  # Skip files without suffixes
            return

        md = self._get_metadata(f.path)

        for md_key, md_val in md.items():
            if md_key not in self.key_index:
                self.key_index[md_key] = {}
            self.key_index[md_key][f.path] = md_val
            self.file_index[f.path][md_key] = md_val