input: Write a python function to extract elements containing the substring 'foo' from a list.
  assert foo(['foobar', 'hello', 'foobaz', 'world', 'foo']) == ['foo', 'foo', 'foo']
  assert foo(['hello', 'world']) == []
  assert foo(['food', 'football', 'fool', 'frame']) == ['foo', 'foo', 'foo']
output: def foo(lst):
      return [item for item in lst if 'foo' in item]