open class Shape {
    open val vertexCount: Int = 0
    open fun draw() { /*...*/ }
    fun fill() { /*...*/ }
}

class Rectangle() : Shape() {
    override fun draw() { /*...*/ }        // overrides method
    override val vertexCount = 4        // overrides property, can be set later
}

class Losange(override val vertexCount: Int = 4) : Shape 
// Always has 4 vertices, can't be set later