protected void addExpression(CrosstabMeasureBuilder<?> crosstabMeasure) {
   Validate.notNull(crosstabMeasure, "crosstabMeasure must not be null");
   this.expressions.add(Expressions.crosstabValue(crosstabMeasure));
 }
 protected void addExpression(AbstractCrosstabGroupBuilder<?, ?, ?> crosstabGroup) {
   Validate.notNull(crosstabGroup, "crosstabGroup must not be null");
   this.expressions.add(Expressions.crosstabValue(crosstabGroup));
 }