title: Scala PartialFunction can be Monoid?
body: <p>I thought <a href="https://github.com/scala/scala/blob/v2.9.1/src/library/scala/PartialFunction.scala">PartialFunction</a> can be <a href="https://github.com/scalaz/scalaz/blob/v6.0.4/core/src/main/scala/scalaz/Monoid.scala">Monoid</a>. Is my thought process correct ?
  For example, </p>
  
  <pre class="lang-scala prettyprint-override"><code>import scalaz._
  import scala.{PartialFunction =&gt; --&gt;}
  
  implicit def partialFunctionSemigroup[A,B]:Semigroup[A--&gt;B] = new Semigroup[A--&gt;B]{
    def append(s1: A--&gt;B, s2: =&gt; A--&gt;B): A--&gt;B = s1.orElse(s2)
  }
  
  implicit def partialFunctionZero[A,B]:Zero[A--&gt;B] = new Zero[A--&gt;B]{
    val zero = new (A--&gt;B){
      def isDefinedAt(a:A) = false
      def apply(a:A) = sys.error("error")
    }
  }
  </code></pre>
  
  <p>But current version Scalaz(6.0.4) is not included it.  Is there a reason for something not included ? </p>