> iex --erl "+S 1" -S mix *
iex(1)> :observer.start()
:ok
iex(2)> Demo.fibonacci(20)
6765
iex(3)> Demo.fibonacci(99)
218922995834555169026
iex(4)> Demo.fibonacci(-1)
…
* Start the Erlang VM with a single process scheduler to demonstrate the
infinite loop more explicitly.