TODO: Finsish section on built-in Erlang and LFE functions

[oubiwann]

LFE URL:

• http://lfe.io/tutorial/sequential/bifs.html

Corresponds to Erlang URL

• https://erlang.org/doc/getting_started/seq_prog.html#built-in-functions--bifs-

[joshrotenberg]

Looks like the LFE URL is now https://lfe.io/books/tutorial/sequential/bifs.html