class Opal::Nodes::Args::ExtractPostOptarg


end
post_args = post_args[1..-1]
a = post_args[0]
if post_args.length > 3
becomes something like:
def m(a = 1, b, c, d); end
args_to_keep is the number of required post-arguments
optional post-argument
This node is responsible for extracting a single

def compile 

def compile
  add_temp name
  line "if ($post_args.length > #{args_to_keep}) #{name} = $post_args.shift();"
  return if default_value.children[1] == :undefined
  push "if (#{name} == null) #{name} = ", expr(default_value)
end