RIoW for Array#combination, but it is a single method.

my-combination.rb

require "test/unit"

def combination(a, k)
  return a.map{|n| [n] } if k == 1

  a[0..(a.size - k)].each_with_index.reduce([]) do |c, (n, i)|
    c + combination(a[i.succ..-1], k - 1).map{|r| [n, *r]}
  end
end

class TestCmb < Test::Unit::TestCase
  test 'it returns single size array of array when k = 1' do
    assert do
      combination([1,2,3], 1) == [[1], [2], [3]]
    end
  end

  test 'it returns combinations for minimum condition' do
    assert do
      combination([1,2], 2) == [[1,2]]
    end
  end

  test 'it returns array of combinations for small array' do
    assert do
      combination([1,2,3], 2) == [[1,2], [1,3], [2,3]]
    end
  end

  test 'it returns array of combinations for modarite size array' do
    combination([1,2,3,4,5,6,7], 4) == [[1, 2, 3, 4], [1, 2, 3, 5], [1, 2, 3, 6], [1, 2, 3, 7], [1, 2, 4, 5], [1, 2, 4, 6], [1, 2, 4, 7], [1, 2, 5, 6], [1, 2, 5, 7], [1, 2, 6, 7], [1, 3, 4, 5], [1, 3, 4, 6], [1, 3, 4, 7], [1, 3, 5, 6], [1, 3, 5, 7], [1, 3, 6, 7], [1, 4, 5, 6], [1, 4, 5, 7], [1, 4, 6, 7], [1, 5, 6, 7], [2, 3, 4, 5], [2, 3, 4, 6], [2, 3, 4, 7], [2, 3, 5, 6], [2, 3, 5, 7], [2, 3, 6, 7], [2, 4, 5, 6], [2, 4, 5, 7], [2, 4, 6, 7], [2, 5, 6, 7], [3, 4, 5, 6], [3, 4, 5, 7], [3, 4, 6, 7], [3, 5, 6, 7], [4, 5, 6, 7]]
  end
end