Liskov-1.swift

class Rectangle {
    var width: Double
    var height: Double
    
    init(width: Double, height: Double) {
        self.width = width
        self.height = height
    }
    
    func area() -> Double {
        return width * height
    }
}

class Square: Rectangle {
    
    // Some extra methods that Rectangle does not implement
    
}

let square = Square(width: 2, height: 3) // Is that an actual square?
let squareArea = square.area()
print(squareArea)