<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 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