def collision(self):
        """
        Get or set the collision module.

        Available collision modules are:

        - ``'none'`` (default)
        - ``'direct'``
        - ``'tree'``
        - ``'mercurius'`` 
        - ``'direct'``
        
        Check the online documentation for a full description of each of the modules. 
        """
        i = self._collision
        for name, _i in COLLISIONS.items():
            if i==_i:
                return name
        return i