propertie overriding in Kotlin

Overriding properties works in a similar way to overriding methods; properties declared on a superclass that are then redeclared on a derived class must be prefaced with override, and they must have a compatible type. Each declared property can be overridden by a property with an initializer or by a property with a get method.
```
open class Shape {
    open val vertexCount: Int = 0
}

class Rectangle : Shape() {
    override val vertexCount = 4
}
```
You can also override a val property with a var property, but not vice versa. This is allowed because a val property essentially declares a get method, and overriding it as a var additionally declares a set method in the derived class.
Note that you can use the override keyword as part of the property declaration in a primary constructor.
```
interface Shape {
    val vertexCount: Int
}

class Rectangle(override val vertexCount: Int = 4) : Shape // Always has 4 vertices

class Polygon : Shape {
    override var vertexCount: Int = 0  // Can be set to any number later
}
```
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