I want to invoke method `a` from method of module `B`. How can I do it? I don't want to specify `A::a` every time.

```
module A
  def self.a
    "a"
  end
end

module B
  extend A

  def self.b
    a
  end
end

p B::b  # => undefined local variable or method `a' for B:Module

```