>>> # Fibonacci series:
   ... # the sum of two elements defines the next

   >>> # Measure some strings:
   ... words = ['cat', 'window', 'defenestrate']

   >>> # Now call the function we just defined:
   ... fib(2000)

   >>> # Tuples may be nested:
   ... u = t, (1, 2, 3, 4, 5)

   >>> # Tuples are immutable:
   ... t[0] = 88888

   >>> # but they can contain mutable objects:
   ... v = ([1, 2, 3], [3, 2, 1])

   >>> # Demonstrate set operations on unique letters from two words
   ...

   >>> # The repr() of a string adds string quotes and backslashes:
   ... hello = 'hello, world\n'

   >>> # The argument to repr() may be any Python object:
   ... repr((x, y, ('spam', 'eggs')))

   >>> # 3 times 'un', followed by 'ium'
   >>> 3 * 'un' + 'ium'

   >>> # replace some values
   >>> letters[2:5] = ['C', 'D', 'E']

   >>> # now remove them
   >>> letters[2:5] = []

   >>> # clear the list by replacing all the elements with an empty list
   >>> letters[:] = []

   >>> # create a new list with the values doubled
   >>> [x*2 for x in vec]

   >>> # filter the list to exclude negative numbers
   >>> [x for x in vec if x >= 0]

   >>> # apply a function to all the elements
   >>> [abs(x) for x in vec]

   >>> # call a method on each element
   >>> freshfruit = ['  banana', '  loganberry ', 'passion fruit  ']

   >>> # create a list of 2-tuples like (number, square)
   >>> [(x, x**2) for x in range(6)]

   >>> # the tuple must be parenthesized, otherwise an error is raised
   >>> [x, x**2 for x in range(6)]

   >>> # flatten a list using a listcomp with two 'for'
   >>> vec = [[1,2,3], [4,5,6], [7,8,9]]

    >>> # We can check that the file has been automatically closed.
    >>> f.closed

   >>> # dates are easily constructed and formatted
   >>> from datetime import date

   >>> # dates support calendar arithmetic
   >>> birthday = date(1964, 7, 31)