Delete all versions of the item from the current library which aren't
        currently part of some snapshot.

        Parameters
        ----------
        symbol : `str`
            symbol name to delete

def delete(self, symbol):
        """
        Delete all versions of the item from the current library which aren't
        currently part of some snapshot.

        Parameters
        ----------
        symbol : `str`
            symbol name to delete
        """
        logger.info("Deleting data item: %r from %r" % (symbol, self._arctic_lib.get_name()))
        # None is the magic sentinel value that indicates an item has been deleted.
        sentinel = self.write(symbol, None, prune_previous_version=False, metadata={'deleted': True})
        self._prune_previous_versions(symbol, 0)

        # If there aren't any other versions, then we don't need the sentinel empty value
        # so delete the sentinel version altogether
        snapped_version = self._versions.find_one({'symbol': symbol,
                                                   'metadata.deleted': {'$ne': True}})
        if not snapped_version:
            self._delete_version(symbol, sentinel.version)
        assert not self.has_symbol(symbol)