code: def is_variable(value):
      
      if isinstance(value, string_types):
          return (value.startswith('@') or value.startswith('-@'))
      elif isinstance(value, tuple):
          value = ''.join(value)
          return (value.startswith('@') or value.startswith('-@'))
      return False
docs: Check if string is LESS variable
      args:
          value (str): string
      returns:
          bool