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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def reverse_ad(node, wrt, preserve_result, check_dims):
 """Perform reverse-mode AD on an AST. This function analyses the AST to determine which variables are active and proceeds by taking the naive derivative. Before returning the primal and adjoint it annotates push and pop statements as such. Args: node: A `FunctionDef` AST node. wrt: A tuple of argument indices with respect to which we take the derivative. preserve_result: A boolean indicating whether the generated derivative function should also return the original return value. check_dims: A boolean indicating whether the seed derivatives should have their dimensions checked to match their primal counterpart. Returns: mod: A `Module` node containing the naive primal and adjoint of the function which can be fed to the `split` and `joint` functions. required: A list of tuples of functions and argument indices. These functions were called by the function but did not have an adjoint. """

  if not isinstance(node, gast.FunctionDef):
    raise TypeError
  # Activity analysis
  cfg.forward(node, cfg.Active(wrt))

  ad = ReverseAD(wrt, preserve_result, check_dims)
  pri, adj = ad.visit(node)
  mod = gast.Module(body=[pri, adj])
  mod = annotate.find_stacks(mod)
  return mod, ad.required, ad.stack