title: Merge maps in scalaz with a complex (double) operation
body: <p>I am using a map to associate certain values with a tuple (Int, Double) where the int is the order they appeared and the double the number of times they show (it is not, but is clearer like this using int and double to distinguish)</p>
  
  <p>The tricky part is that I want to use different monoids for each element of the tuple, for the int I want to keep the min value, to remember first appearance, while for the double I want to use the addition monoid
  So for an existing key we have:</p>
  
  <pre><code>val map1 = Map("a" -&gt; (1, 5.0), "b" -&gt; (2, 4.0), "c" -&gt; (3, 8.0))
  val map2 = Map("b" -&gt; (4, 1.0))
  
  val merge = map1.toMap |+| map2.toMap
  // Map(a -&gt; (1, 5.0), b -&gt; (2, 5.0), c -&gt; (3, 8.0))
  </code></pre>
  
  <p>And for a new key we have:</p>
  
  <pre><code>val map2 = Map("d" -&gt; (4, 1.0))
  
  val merge2 = map1.toMap |+| map2.toMap
  // Map(a -&gt; (1, 5.0), b -&gt; (2, 4.0), c -&gt; (3, 8.0), d -&gt; (4, 1.0))
  </code></pre>
  
  <p>I can not find a way do this, i can obviously use the addition monoid, and i can use the minval one, but i can not see how to combine them.
  Any help appreciated! thanks</p>