I think the following would make a good example:

Create an example function which traverses the SyntaxTree, and renames all signals it encounters to be suffixed with "_visited".

It could even be written in the style of a unit test, with the following input and output:

Example Input

module RefModule (
  input in1,
  input in2,
  output logic out
);

  assign out = in1 & ~in2;

endmodule

Example Output

module RefModule (
  input in1_visited,
  input in2_visited,
  output logic out_visited
);

  assign out_visited = in1_visited & ~in2_visited;

endmodule

Additional context
I'd be happy to help finish this in a PR if you can just give a crude example of how this is accomplished.