Xpath to join all resulting nodes with string seperator

I want to join the values of all the nodes that match an expression and have them separated by a comma. For example, the following xml should return "num1,num2,num3"

<products>
  <product>
    <id>1</id>
    <other>y</other>
    <notarget>x</notarget>
    <target>num1</target>
    <target>num2</target>
    <target>num3</target>
  </product>
</products>

The xpath /[name()='products']/[name()='product']/*[name()='target'] should get the elements but I just don't understand how to join them