code: public NumberExpression<Double> avg() {
          if (avg == null) {
              avg = Expressions.numberOperation(Double.class, Ops.AggOps.AVG_AGG, mixin);
          }
          return avg;
      }
docs: Create a {@code avg(this)} expression
  
  <p>Get the average value of this expression (aggregation)</p>
  
  @return avg(this)