private static Operator get(String operatorString) {
            Operator operator=operators.get(operatorString);
            if (operator==null)
                throw new IllegalArgumentException("Unknown operator '" + operatorString + "'");
            return operator;
        }
A condition which is true of the <i>values</i> of its two subconditions are true
and both have the same value

@author bratseth
container-search/src/main/java/com/yahoo/prelude/semantics/rule/ComparisonCondition.java