code: public final BooleanExpression isEmpty() {
          if (empty == null) {
              empty = Expressions.booleanOperation(Ops.COL_IS_EMPTY, mixin);
          }
          return empty;
      }
docs: Create a {@code this.isEmpty()} expression
  
  <p>Evaluates to true, if this has no elements.</p>
  
  @return this.isEmpty()