Apply feature creation functions only where they don’t already exist in combined-features.

Combine features from the forms for which features don’t already exist in features, which is a map with functions as keys and features already created with that function as the value of the respective function.

Limitations: each form must be a function call and not a macro or any other special form.

Create a lexical context of combined features. Wrap this around all of your feature generation function calls. To be used with combine-features.