===== The <rule> Element =====
==== Definition and Usage ====

The [[rule|<rule>]] element defines simple rules for defining either alternative
components or alternative descriptors.

The //id// attribute is the rule identifier.  A rule is established by
comparing (//comparator// attribute) a variable (//var// attribute)
value with an absolute value.  The variable shall be a property of the
settings node ([[media|<media>]] element of application/x-ncl-settings type),
that is, the //var// attribute must have as its valuea property name of
the [[media|<media>]] element of "application/x-ncl-settings" type.

==== Syntax ====
^ Element ^ Attributes ^ Content ^ Parent ^
| [[rule|<rule>]] | //id//, __//var//__, __//comparator//__, __//value//__ | -- | [[rulebase|<ruleBase>]] or [[compositerule|<compositeRule>]] |
==== Attributes ====

^ Name ^ Value ^ Description ^
| //id// | It may receive any string value that begins with a letter or an underscore and that only contains letters, digits, <".">, and <"_">. | The //id// attribute uniquely identifies the element within a document. |
| //var// | It may receive any string value that begins with a letter or an underscore and that only contains letters, digits, <".">, and <"_">. | The //var// attribute uniquely identifies a property name of the settings node ([[media|<media>]] element of "application/x-ncl-settings" type). |
| //comparator// | "eq", "ne", "gt", "lt", "gte", or "lte". | The //comparator// attribute compares the value of the property specified by the //var// attribute with the absolute value specified in the //value// attribute. |
| //value// | String. | The //value// attribute defines a value to be compared with the property value referred by the //var// attribute. |