merge

def empty?(e)
  e.peek
  false
rescue
  true
end

def merge(*src)
  src = src.map{|s| s.to_enum}
  while true
    src = src.delete_if{|s| empty?(s)}
    return if src.empty?
    it = src.min_by {|e| e.peek.to_i}
    yield(it.next.to_i)
  end
end

if __FILE__ == $0
  merge(File.open('aaa'), File.open('bbb')){|x| p x}
  merge(File.open('aaa'), [2,5,11,12,13,90]){|x| p x}
end