code: def to_dict(self):
          
          make = lambda pair: (pair[0], pair[1].to_dict())
          return dict(
              variables=dict(make(v) for v in self.variables.items()),
              properties=dict(make(p) for p in self.properties.items()),
              roles=dict(make(r) for r in self.roles.items()),
              predicates=dict(make(p) for p in self.predicates.items())
          )
docs: Return a dictionary representation of the SemI.