An attribute is represented by a single cell. The first element of the cell references the attribute key, the second value the corresponding value.

AttributeKey and AttributeValue are string values, except when used internally. In this case, the attribute key may be a symbol, e.g. *ZSX-ID*.

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”.