ContextOps.kt

interface Group<T> {
    val id: T // identity
    operator fun T.plus(that: T): T // group operation
    operator fun T.unaryMinus(): T // inverse
}

inline class Residue(val x: Int)

// Group of residues modulo n
class Mod(val n: Int): Group<Residue> {
    override val id: Residue = Residue(0)
    override fun Residue.plus(that: Residue): Residue = Residue((x + that.x) % n)
    override fun Residue.unaryMinus(): Residue = Residue((n - x) % n)
}

// Contextual functions
context(Group<T>)
fun <T> Collection<T>.sum(): T {
    var a = id
    for (x in this) a += x
    return a
}

// Contextual operator
context(Group<T>)
operator fun <T> T.minus(that: T): T = this + (-that)

fun main() {
    with(Mod(11)) {
        println(Residue(3) - Residue(5))
    }
}