def _select_theory(theories):
        """Return the most likely spacing convention given different options.

        Given a dictionary of convention options as keys and their occurrence
        as values, return the convention that occurs the most, or ``None`` if
        there is no clear preferred style.
        """
        if theories:
            values = tuple(theories.values())
            best = max(values)
            confidence = float(best) / sum(values)
            if confidence > 0.5:
                return tuple(theories.keys())[values.index(best)]