sandwhich principle (ruby koans)

hello=2C  I'm working on ruby koans.
Now I have to do some sandwhich code. The exercise looks like this : requir=
e File.expand_path(File.dirname(__FILE__) + '/edgecase')class AboutSandwich=
Code < EdgeCase::Koan  def count_lines(file_name)
    file =3D open(file_name)
    count =3D 0
    while line =3D file.gets
      count +=3D 1
    end
    count
  ensure
    file.close if file
  end  def test_counting_lines
    assert_equal 4=2C count_lines("example_file.txt")
  end  # ------------------------------------------------------------------=
  def find_line(file_name)
    file =3D open(file_name)
    while line =3D file.gets
      return line if line.match(/e/)
    end
  ensure
    file.close if file
  end  def test_finding_lines
    assert_equal "test\n"=2C find_line("example_file.txt")
  end  # ------------------------------------------------------------------
  # THINK ABOUT IT:
  #
  # The count_lines and find_line are similar=2C and yet different.
  # They both follow the pattern of "sandwich code".
  #
  # Sandwich code is code that comes in three parts: (1) the top slice
  # of bread=2C (2) the meat=2C and (3) the bottom slice of bread.  The
  # bread part of the sandwich almost always goes together=2C but
  # the meat part changes all the time.
  #
  # Because the changing part of the sandwich code is in the middle=2C
  # abstracting the top and bottom bread slices to a library can be
  # difficult in many languages.
  #
  # (Aside for C++ programmers: The idiom of capturing allocated
  # pointers in a smart pointer constructor is an attempt to deal with
  # the problem of sandwich code for resource allocation.)
  #
  # Consider the following code:
  #  def file_sandwich(file_name)
    file =3D open(file_name)
    yield(file)
  ensure
    file.close if file
  end  # Now we write:  def count_lines2(file_name)
    file_sandwich(file_name) do |file|
      count =3D 0
      while line =3D file.gets
        count +=3D 1
      end
      count
    end
  end  def test_counting_lines2
    assert_equal 4=2C count_lines2("example_file.txt")
  end  # ------------------------------------------------------------------=
  def find_line2(file_name)
    # Rewrite find_line using the file_sandwich library function.
  end  def test_finding_lines2
    assert_equal __=2C find_line2("example_file.txt")
  end  # ------------------------------------------------------------------=
  def count_lines3(file_name)
    open(file_name) do |file|
      count =3D 0
      while line =3D file.gets
        count +=3D 1
      end
      count
    end
  end  def test_open_handles_the_file_sandwich_when_given_a_block
    assert_equal __=2C count_lines3("example_file.txt")
  endend
But I don't get the principle.Can anyone explain this to me ? Roelof
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