docs: def variable_names(self):
          """
          Get all variable names required for this query
          """
          if self._variable_names is None:
  
              if self._operator is None:
                  if self._operands is None:
                      self._variable_names = tuple()
                  else:
                      self._variable_names = self._get_variable_names(self._operands)
  
              elif self._operator == 'NOT':
                  self._variable_names = self._operands.variable_names
  
              else:
                  v = list()
                  for op in self._operands:
                      v.extend(op.variable_names)
                  self._variable_names = tuple(set(v))
  
          return self._variable_names