This pass descends the entire SXML program and attaches a property to each Lambda PrimExp.t in the program. Then, you can use lambdaFree and lambdaRec to get free variables of that Lambda.
Details and Notes
For Lambda-s bound in a Fun dec, lambdaFree gives the union of the frees of the entire group of mutually recursive functions. Hence, lambdaFree for every Lambda in a single Fun dec is the same. Furthermore, for a Lambda bound in a Fun dec, lambdaRec gives the list of other functions bound in the same dec defining that Lambda.
val rec f = fn x => ... y ... g ... f ... and g = fn z => ... f ... w ...
lambdaFree(fn x =>) = [y, w] lambdaFree(fn z =>) = [y, w] lambdaRec(fn x =>) = [g, f] lambdaRec(fn z =>) = [f]