data = [['a','b'],['c','d','e'],['f','g','h','i']]

def combine(a, b)
  if b.is_a?(Array) && !b.empty?
    tmp = []
    c = b.shift
    a.each_with_index do |aa, i|
      c.each do |cc|
        if aa.is_a?(Array)
          tmp.push aa.clone.push(cc) # may be a ruby bug: must use clone
        else
          tmp.push [aa, cc]
        end
      end
    end

    if !b.empty?
      return combine(tmp, b)
    else
      return tmp
    end
  end
end

p "result:"
result = combine(data.shift, data)
p result
p result.size