15. The apparatus of claim 1, wherein the query engine is programmed to evaluate an operator having an operand of unknown value.
15. The apparatus of claim 1, wherein the query engine is programmed to evaluate an operator having an operand of unknown value. 16. The apparatus of claim 15, wherein the operator is selected from the group consisting of logical operators, comparison operators, quantifier operators, aggregator operators, selector operators, field existence operators, and arithmetic operators.