Having a standard SQL extension definitely eases adoption and ability to migrate from one tool to another. But rather than adding and adding features to SQL; we

Having a standard SQL extension definitely eases adoption and ability to migrate from one tool to another. But rather than adding and adding features to SQL; we maybe need a simpler way to extend/compose features. On that point, I see a contradiction in the future work section of the paper where it said that "Experience has also shown that pre-built solutions are never sufficient for all use cases; ultimately, users should be able to utilize the power of SQL to describe their own custom-windowing TVFs". But, how can a SQL user add its own custom operator?