code: def set_var(self, name, value, user=None):
          
          # Set a user variable
          if user is not None:
              if user not in self._users:
                  raise UserNotDefinedError
  
              self._users[user].set_var(name, value)
              return
  
          # Set a global variable
          self._global_vars[name] = value
docs: Set a global or user variable
          :param name: The name of the variable to set
          :type  name: str
  
          :param value: The value of the variable to set
          :type  value: str
  
          :param user: If defining a user variable, the user identifier
          :type  user: str
  
          :raises UserNotDefinedError: The specified user does not exist