Coding challenge: Force non-equality - runtime

I’m curious if anyone would be interesting in giving and trying an occasional coding challenge (perhaps a new tag or category could be made if it gets popular). These would not be ones like Exercism has or so, but strictly challenges to try to solve language-specific tasks that you may never run across in real-life. Let’s do a simple task as an example:

Force non-equality - runtime

Given this code:

defmodule Testing do
  def f() do
    # Put 1-expression
  end

  def test() do
    false = ( f() == f() )
    true
  end
end

Fill in the # Put 1-expression here with code to make the test pass where calling Testing.test() returns true.