Quick reference to the new features in Ruby 2.1.

### Named arguments with defaults

    # length is required
    def pad(num, length:, char: "0")
      num.to_s.rjust(length, char)
    end

    pad(42, length: 6) #=> "000042"
    pad(42) #=> #<ArgumentError: missing keyword: length>

### Module.prepend

    prepend(
      Module.new do
        define_method ...
      end
    )

### References

- http://globaldev.co.uk/2013/03/ruby-2-0-0-in-detail
- http://globaldev.co.uk/2014/05/ruby-2-1-in-detail

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