Transform the body of function.

Traverses the body of fn and rewrite it based on callbacks.

depth_first_rewrite_function(fn, callbacks, topdown = list(),
  wflags = warning_flags(), ...)

Arguments

fn	A (closure) function.
callbacks	List of callbacks to apply.
topdown	A list of additional information that will be considered top-down in the traversal.
wflags	Warning flags, see `warning_flags()`.
...	Additional data that will be passed along to callbacks.

A new function similar to fn but with a transformed body.