<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def add_resource(self, resource):
 """Add a resource to the list of interesting resources"""

        if resource.exists():
            self.resources[resource] = self.timekeeper.get_indicator(resource)
        else:
            self.resources[resource] = None