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](00001012930500).

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”.