I'm confused about the Scala parser combinator libraries

Problem.scala

package wolv.parserlibproblem

import util.parsing.ast.AbstractSyntax
import util.parsing.combinator.syntactical.TokenParsers
import util.parsing.combinator.lexical.Lexical
import util.parsing.combinator.token.Tokens

trait SimpleTokens extends Tokens {
  case class Foo extends Token
  case class Bar extends Token
}

class SimpleLex extends Lexical with SimpleTokens {
}

trait SimpleAst extends AbstractSyntax {
  sealed trait Exp extends Element
  case class Num() extends Exp
  case class Str() extends Exp
}

class Parser extends TokenParsers with SimpleAst {
  type Tokens = SimpleLex

  def parse = Num()
}

class PrettyPrinter extends SimpleAst {
  def print(x: Exp) = 42
}

class Problem extends Application {
  val parser = new Parser
  val prettyPrinter = new PrettyPrinter

  println(prettyPrinter.print(parser.parse))
}

/*
Problem.scala:38: error: type mismatch;
[INFO]  found   : Problem.this.parser.Num
[INFO]  required: Problem.this.prettyPrinter.ast.Exp
[INFO]   println(prettyPrinter.print(parser.parse))
*/