input: Write a python function to concatenate a list of words into a single string.
  assert reduce(['hello', 'world']) == 'helloworld'
  assert reduce(['foo', 'bar', 'baz']) == 'foobarbaz'
  assert reduce(['a', 'b', 'c', 'd']) == 'abcd'
output: def reduce(words):
      return ''.join(words)