Theorem screening question

smoothen.rb

# An idiomatic Ruby mixin to Array-like classes.
module ArrayX
  # A re-implementation of the native `Array#flatten` method.
  # Returns a one-dimensional flattened version of `self`,
  # or flattens it to a degree, specified by `level`.
  #
  # @param level [Number] level of recursion
  # @return [Array] flattened array
  def smoothen(level = -1)
    return self if level == 0
    
    inject([]) do |acc, x|
      if x.respond_to? :smoothen
        acc + x.smoothen(level - 1)
      else
        acc + [x]
      end
    end
  end
end

if __FILE__ == $0
  require 'test/unit'

  Array.include(ArrayX)

  class TestArrayX < Test::Unit::TestCase
    def test_smoothening_of_arrays
      assert_equal [], Array.new([]).smoothen
      
      arr = Array.new([[0], 1, [2, [3, [4, [5], 6], 7], 8], 9])
      assert_equal [[0], 1, [2, [3, [4, [5], 6], 7], 8], 9], arr.smoothen(0)
      assert_equal [0, 1, 2, [3, [4, [5], 6], 7], 8, 9], arr.smoothen(1)
      assert_equal [0, 1, 2, 3, [4, [5], 6], 7, 8, 9], arr.smoothen(2)
      assert_equal [0, 1, 2, 3, 4, [5], 6, 7, 8, 9], arr.smoothen(3)
      assert_equal [0, 1, 2, 3, 4, 5, 6, 7, 8, 9], arr.smoothen(4)
      assert_equal [0, 1, 2, 3, 4, 5, 6, 7, 8, 9], arr.smoothen(5)
      assert_equal [0, 1, 2, 3, 4, 5, 6, 7, 8, 9], arr.smoothen(-1)
      assert_equal [0, 1, 2, 3, 4, 5, 6, 7, 8, 9], arr.smoothen
    end
  end
end