Thanks for your explanation about how to use external variables! I'm interested if you have more resources on the recommended ways to manage external variables in the operations. I also had seen https://uwdata.github.io/arquero/api/#addFunction, but I could not make it work.

Since writing this, I've come to see that the recommend way is to bind parameters to the table, which are then available for computation through a special variable inside functions $. Defining separate ops is also possible, but not always necessary. So for instance, given my_name = "Wesleyan University" then table.filter(d => op.equal(d.INSTNM, my_name)).view() will not work, but table.params({name: my_name}).filter((d, $) => op.equal(d.INSTNM, $.name)).view() works nicely!

Thanks a lot! It's much easier to think about for me, even if the other way is also elegant.

Adding to the conversation for anyone else researching this issue; as detailed [in this forum discussion](https://talk.observablehq.com/t/dynamic-rollup-object-for-arquero/4721). I've encountered a [use case](https://observablehq.com/@mariodelgadosr/dow-jones-industrial-average-correlation-analysis-with-ar#rollupObj) where the only alternative was to use the [eval](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/eval) method to dynamically create the properties for data-driven rollup processing.