input: Write a function to delete a given key from a dictionary if the key exists in the dictionary. The function should return None and the dictionary should be modified in place.
  
  Input:
  dictionary = {'foo': 1, 'bar': 2, 'baz': 3}
  key = 'baz'
  
  Output:
  {'foo': 1, 'bar': 2}
  
  def delete_key(dic, k):
    if k in dic:
      del dic[k]
  
  assert delete_key({'foo': 1, 'bar': 2, 'baz': 3}, 'baz')==None
  assert {'foo': 1, 'bar': 2, 'baz': 3} == {'foo': 1, 'bar': 2}
output: def delete_key(dic, k):
    if k in dic:
      del dic[k]