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trait Ring[T <: Ring[T]] {
def +(that: T): T
def *(that: T): T
}
class A extends Ring[A] {
def +(that: A) = new A
def *(that: A) = new A
}
class Poly[C <: Ring[C]](val c: C) extends Ring[Poly[C]] {
def +(that: Poly[C]) = new Poly(this.c+that.c)
def *(that: Poly[C]) = new Poly(this.c*that.c)
}
implicit def coef2poly[C <: Ring[C]](c: C) = new Poly(c)
val a = new A
val x = new Poly(new A)
x+a // works
a+x // works
val y = new Poly(new Poly(new A))
x+y*x // works
x*y+x // works
y*x+x // works
x+x*y
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