Fitted column transforms as a composable monoid. A Transform is a list of named output expressions; s <> t means "apply s, then t", fusing t's references to s's outputs by simultaneous substitution. applyTransform runs one against a frame; compileThrough folds a transform into a model's prediction expression so the result is a single expression over the raw inputs.

Every right-hand side must be row-wise (no aggregation/window), and within one transform each expression reads the original frame.