[m2] dependency versions

[Xavier Toth]

In http://www.ibiblio.org/maven2/activemq/activemq/3.1/activemq-3.1.pom a
number of the dependencies contain versions like the following. The
dependencies don't download is it valid to use a variable?

    <dependency>
      <groupId>xstream</groupId>
      <artifactId>xstream</artifactId>
      <version>${xstream_version}</version>
    </dependency>