Object-oriented "chop"

Dave (you know, one of the pragmatic programmers) provides mini exercices, so-called Katas, on his blog. I have discovered them recently although he put the Katas up there in 2003. Following you find my fifth implementation of the chop-algorithm of Kata 2:

class Array
  def middle
    size / 2
  end
  
  def partition(target)
    in_lower?(target) ? self[0 ... middle] : self [middle .. -1]
  end
  
  def in_lower?(target)
    self[middle] > target
  end
  
  def offset(target)
    in_lower?(target) ? 0 : middle
  end
  
  def chop(target, offset = 0)
    if (size <= 1)
      self[0] == target ? offset : -1
    else
      partition(target).chop(target, offset(target) + offset)
    end
  end
end

def chop(target, items)
  items.chop(target)
end

Did I already mention I really like Ruby's open classes? :) My implementation was initially inspired by Jim Weirich's recursive implementation.