A rule node is a closed definition node that defines constraints via aspect rules (see formula component). A rule node defines zero or more rule sets, that is, sets of aspect rules. Each rule set may specify a tag. At most, one of these rule sets may omit the tag. This untagged rule set is always displayed before all tagged rule sets. An empty untagged rule set is not displayed if at least one tagged rule set is present. A rule node provides two Boolean properties, abstract and merge, as icons. The screenshot below shows a rule node without aspect rules.