code: public NumberExpression<T> sum() {
          if (sum == null) {
              sum = Expressions.numberOperation(getType(), Ops.AggOps.SUM_AGG, mixin);
          }
          return sum;
      }
docs: Create a {@code sum(this)} expression
  
  <p>Get the sum of this expression (aggregation)</p>
  
  @return sum(this)