docs: def from_string(dir_string):
      '''Returns the correct constant for a given string.
  
      @raises InvalidDirectionError
  
      '''
      dir_string = dir_string.upper()
      if dir_string == UP:
          return UP
      elif dir_string == DOWN:
          return DOWN
      elif dir_string == LEFT:
          return LEFT
      elif dir_string == RIGHT:
          return RIGHT
      else:
          raise InvalidDirectionError(dir_string)