docs: def BindVar(self, var_id, value):
      """Associates a value with given variable.
  
      This can be called multiple times to associate multiple values.
  
      Args:
        var_id: A variable id to bind the values to.
        value: A value to bind to the specified variable.
  
      Raises:
        KeyError: If given variable is not specified in the pattern.
      """
      if var_id not in self._vars:
        raise KeyError(var_id)
  
      self._var_bindings[var_id].append(value)