def _file_in_patch(self, filename, patch):
pc_dir = self.quilt_pc + patch.get_name()<EOL>file = pc_dir + File(filename)<EOL>if not file.exists():<EOL><INDENT>raise QuiltError("<STR_LIT>" % (filename,<EOL>patch.get_name()))<EOL><DEDENT>
Checks if a backup file of the filename in the current patch
        exists and raises a QuiltError if not.