Starts a new group for the given prefix.
        
        :param      prefix | <str>

def beginGroup(self, prefix):
        """
        Starts a new group for the given prefix.
        
        :param      prefix | <str>
        """
        curr = self._stack[-1]
        next = curr.get(prefix, {})
        curr.setdefault(prefix, next)
        self._stack.append(next)