The concordion:echo tag evaluates an expression and inserts
the result into the output HTML.
If the expression "username" evaluates to "jbloggs" and we have the following instrumentation in our specification:
<p>
Username: <span concordion:echo="username" />
</p>
Then we expect the following output:
<p>
Username: <span concordion:echo="username">jbloggs</span>
</p>