Another way of describing complex relationships between groups of RootEntitiy’s instances is to use expressions.<br/>In this case we use the sub-class of RootEntityGroupChoice to carry attributes usually carried by RootEntityGroupRelationship (min, max and default) and theRootEntityGroupMemeberExpression sub-class for defining the expression.<br/>Like this you can specify mathematical expressions composed of variables (represented by the members), operators and delimiters.<br/>