Chapter 14: Show default arguments for functions

[radar]

In Chapter 13, we will show the documentation for Map.get/2, which has this title:

def get(map, key, default \\ nil)

The default \\ nil syntax is new to readers as of this point. It should probably be introduced in Chapter 6.

[radar]

Except:

a = fn (a \\ nil) -> "Hello #{a}" end
** (CompileError) iex:26: anonymous functions cannot have optional arguments
    (stdlib) lists.erl:1354: :lists.mapfoldl/3
    (elixir) src/elixir_fn.erl:19: :elixir_fn.expand/3

Probably something to show in Ch14 then.