input: Write a function to create a dictionary that maps each key x from a given list to its square value x^2. If the list is empty, return an empty dictionary.
  assert square_dict([1, 2, 3, 4, 5])=={1: 1, 2: 4, 3: 9, 4: 16, 5: 25}
  assert square_dict([-1, -2, -3])=={-1: 1, -2: 4, -3: 9}
  assert square_dict([0])=={0: 0}
  assert square_dict([])=={}
output: def square_dict(lst):
      return {x: x**2 for x in lst}