input: Write a function to create a class in Python that stores and processes data for a Car object. The class should have an initializer method to set the make, model, color, and mileage of the car. It should also have methods to accelerate and decelerate the car by setting its speed.
  
  class Car:
      def __init__(self, make, model, color, mileage):
          self.make = make
          self.model = model
          self.color = color
          self.mileage = mileage
      
      def accelerate(self, speed):
          self.speed = speed
  
      def decelerate(self, speed):
          self.speed = speed
output: 
  class Car:
      def __init__(self, make, model, color, mileage):
          self.make = make
          self.model = model
          self.color = color
          self.mileage = mileage
          self.speed = 0
      
      def accelerate(self, speed):
          self.speed += speed
  
      def decelerate(self, speed):
          self.speed -= speed
  