input: Write a python function to create a class called UniqueSet that allows adding, removing, checking for elements, and checking the size of the set.
  assert UniqueSet().size() == 0
  us = UniqueSet()
  us.add(1)
  us.add(2)
  assert us.contains(1) == True
  assert us.contains(3) == False
  us.remove(1)
  assert us.contains(1) == False
  assert us.size() == 1
output: class UniqueSet:
      def __init__(self):
          self.elements = set()
          
      def add(self, value):
          self.elements.add(value)
          
      def remove(self, value):
          self.elements.discard(value)
          
      def contains(self, value):
          return value in self.elements
          
      def size(self):
          return len(self.elements)