kotlin operator overloading - ref: https://kotlinlang.org/docs/reference/operator-overloading.html to find allowed operators to overload

operatorOverLoading.kt

fun main() {
    val bluePen = Pen(inkColor = "Blue")
    bluePen.showInkColor()

    val blackPen = Pen(inkColor = "Black")
    blackPen.showInkColor()

    val blueBlackPen = bluePen + blackPen
    blueBlackPen.showInkColor()
}

 operator fun Pen.plus(otherPen: Pen):Pen{
     val ink = "$inkColor, ${otherPen.inkColor}"
     return Pen(inkColor = ink)
 }

 data class Pen(val inkColor:String){
     fun showInkColor(){ println(inkColor)}
 }