For the library, you are right, it has to support any tabular structure — but that’s what even ricks are for, to encode that there are parts that you don’t know

For the library, you are right, it has to support any tabular structure — but that’s what even ricks are for, to encode that there are parts that you don’t know until use. When using the library, though, there is some level of assumed structure (otherwise your code wouldn’t work since it would be accessing fields that don’t exist. Sure data is messy in real life, so you encode that messy-ness into your structure as variants or optional that you then check, and ideally are forced to check so that you can’t silently let errors slip in). This works in statically typed languages right now using generics, templates, types that aren’t inferred until the library is used, or whatever the specific language has to deal with this.