Compilation error

CKnot.scala

/*
So here's the error which seems to be straight forward. That said, I've tried a bunch of things and this is the "bets" I've been
able to do:

[error] CKnot.scala:18: Cannot construct a collection of type That with elements of type B based on a collection of type C[A].
[error] 			def apply[A, C[A] <: GenTraversableLike[A, C[A]]](ma: C[A], f: A => C[B]) = ma flatMap f
[error] 
[error] CKnot.scala:26: Cannot construct a collection of type That with elements of type B based on a collection of type C[A].
[error] 				a <- ma
[error] 

...and it continues
*/

import scala.collection.{GenTraversable, GenTraversableLike}

trait CKnot[C[+_], B]{
	type R

	def apply[A](ma: C[A], f: A => B): C[R]
}

trait CKnotCompanion extends LowPriorityCKnot{
	def apply[B](implicit knot: CKnot[C, B]): Aux[C, B, knot.R] = knot

	implicit def Cbase[B]: Aux[C, C[B], B] =
		new CKnot[C, C[B]]{
			type R = B

			def apply[A](ma: C[A], f: A => C[B]) = ma flatMap f
		}

	implicit def opt[B]: Aux[C, Option[B], B] =
		new CKnot[C, Option[B]]{
			type R = B

			def apply[A](ma: C[A], f: A => Option[B]) = for{
				a <- ma
				v <- f(a)
			} yield v
		}
}

trait LowPriorityCKnot extends LowestPriorityCKnot{
	implicit def trav[C0[_] <: GenTraversable[_],B]: Aux[C, C0[B], C[B]] =
		new CKnot[C, C0[B]]{
			type R = C0[B]

			def apply[A](ma: C[A], f: A => C0[B]) = (ma map f) filter (_.nonEmpty)
		}
}

trait LowestPriorityCKnot{
	type C[X] <: GenTraversableLike[X, C[X]]

	type Aux[G[_], B, R0] = CKnot[G, B]{ type R = R0 }

	implicit def any[B]: Aux[C, B, B] =
		new CKnot[C, B]{
			type R = B

			def apply[A](ma: C[A], f: A => B) = ma map f
		}
}