An attribute is represented by a single cell. The first element of the cell references the attribute key, the second value the corresponding value.
AttributeKeyValue = ( AttributeKey . AttributeValue ).
AttributeKey and AttributeValue are string values.
An empty key denotes the generic attribute.
A key with the value "-" specifies the default attribute. In this case, the attribute value is not interpreted.
Some examples:
() represents the absence of attributes,
(("-" . "")) represent the default attribute,
(("-" . "") ("" . "syntax")) adds the generic attribute with the value syntax,
(("lang" . "en")) denotes the attribute key lang with a value en.