docs: def check(self):
          """Check if it is a valid RADL document."""
  
          for i in [f for fs in [self.networks, self.ansible_hosts, self.systems, self.deploys,
                                 self.configures, [self.contextualize]] for f in fs]:
              i.check(self)
  
          snames = [s.name for s in self.systems]
          if len(set(snames)) != len(snames):
              raise RADLParseException("Some System name duplicated.")
  
          nids = [s.id for s in self.networks]
          if len(set(nids)) != len(nids):
              raise RADLParseException("Some Network name duplicated.")
  
          return True