Swann missed a big generalization in limiting it to relative clauses and if clauses. Probly it was the "combined with" metaphor that did it. This is not cooking; you don't get a blend, you get a program.
They could have discovered (or rediscovered, depending on the publication date) the Ross Constraints, probably the most important syntactic discovery in the short history of generative grammar.
Details are available in Ross, J. R Constraints on Variables in Syntax, 1967 .
The basic point that's applicable here is that the rule (aka transformation) that forms (relates) relative clauses from (to) their full form, called Relative Formation, is a rule that, as Ross put it, "operates over a variable", meaning that the movement of the relative pronoun to the front of the relative clause can come from indefinitely far down the chained trees of clauses, as long as it doesn't cross certain types of syntactic construction barriers.
E.g, the relative that (which could also be who or whom) is moved from position as object of see
- the boy that the doctor told his receptionist he never wanted to see ____ again
Question Formation is another rule like that
- Who did the doctor tell his receptionist he never wanted to see ____ again?
Except that there are exceptions.
These rules can't cross islands, which are kinds of constructions like coordinate structures
- Frank cooked dinner and Bill washed up ~ *the dinner that Frank cooked and Bill washed up
and complex noun phrases
- She married the man who wrote that book ~ *the book that she married the man who wrote
These exceptions are the Ross Constraints. They apply to pretty much every English sentence beyond The ball is on the table, which contains no islands.