Subsumption permits to delete more specific tuples by more general tuples. A programmer can express this by declaring a partial-order in the form of a subsumptive clause.
A subsumptive rule has a dominated and a dominating head seperated by
<= and is followed by its rule body. The rule body is defined as in a standard rule. A query plan is optional for a subsumptive rule.
rule ::= atom '<=' atom ':-' disjunction '.' query_plan?