Representing relationship operator used for creation of CompositeConditions.
Author of the BOP is expected to write his own ConditionVisitor which interpret incoming Conditions and transform them into proper URL query or generally process them any way (s)he need. Usually ConditionVisitor.visitCompositeCondition(..) needs to behave differently based on the RelationOperator assigned to particular CompositeCondition.
(static) AND :string
Represents relationship operator AND of CompositeCondition.