<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def variables(self):
 """Sorted list of strings summarizing all variables handled by the |Node| objects"""

        variables = set([])
        for node in self.nodes:
            variables.add(node.variable)
        return sorted(variables)