This restriction in some sense preserves the Hornlog expressivity because a more general content model for Operation would affect the expressivity when these Operations appear in the head of rules.
In Consumer RuleML, the content model for Operation is in some ways more permissive and in other ways more strict - it uses the content model for And, which is
In Reaction RuleML, the content model of Operation follows Rulebase. In the Relax NG of Deliberation, this is
RulebaseFormula.choice |= SimpleFormula-node.choice | And-fo-node.choice | Or-fo-node.choice | Negation-node.choice | NegationAsFailure-node.notallowed | Implication-node.choice | Forall-node.choice | Exists-fo-node.choice
This restriction in some sense preserves the Hornlog expressivity because a more general content model for Operation would affect the expressivity when these Operations appear in the head of rules.
In Consumer RuleML, the content model for Operation is in some ways more permissive and in other ways more strict - it uses the content model for And, which is
AndFormula.choice |= SimpleFormula-node.choice | And-node.choice | Or-node.choice | Negation-node.choice | NegationAsFailure-node.choice | Implication-fo-node.choice | Forall-fo-node.choice | Exists-fo-node.choice